Tip on how to fix rusted through body mounts?

180A is a good general purpose size. It's light enough to weld thin steel reasonably well but also has enough grunt for general fabrication. My 230A Portamig is great on heavier stuff but a bit violent on body work.
Argon with 5% CO2 is popular for thinner metal. I personally use 15% CO2 but that's mainly because I tend to do heavier work. 5% is supposed to run a bit cooler. I use 20L cylinders which last me around 2 years.

By the way if you get a replacement panel you may find you only need a part of it. Replacing the whole crossmember is quite a job so it may be more practical to just replace the bad areas. Check the middle of the crossmember below the door. Mine was quite rotted in that area. It's the lowest point so any water that gets in settles there.

I thought I would give an update to how this went.
The problem I started out with is shown in the picture below.
Unfortunately, after picking away on the other side I managed to poke a hole with a screwdriver, so both sides were shot.

 

It wasn't too difficult to figure out how things went together, so I ordered some new parts. Rear tail mermber, outer tail member and what's called extension back corner, left and right, and a new jack bracket. I also bought a mig welder, and a chinese sandblaster.

In hindsight I would say that the hardest part about this hole thing was to find and drill / grind out the factory spot welds. Especially where it was rusted they were hard to find, and access is a bit restricted here and there. The next challenge was to learn how to weld thin sheet metal. It did not work out very well in the beginning, I only burnt holes. After practicing on some scrap pieces in the garage I managed to get to a level where it wasn't pretty, but I didn't burn holes any more, at least not most of the time.

Here I have removed all in the back. Taken off the rear axle and the tank, sandblasted the rear underside:

 

I put a piece of 4x4 between the frame and the body on each side to get it high enough to work with:

 

Here I put in the rear tailmember. It was a very good fit. Fitting those new parts were actually not difficult.

 

The space for the jack was repaired by a previous owner. It didn't fit very well, so I had to do that again. It's not perfect, but it is rust free.

 

Then I also got some surprises. In the right back corner I found a hole in the floor under the "extension, back corner". It was not visible until I had removed this piece. After I sandblasted the underside I found a larger are to be rusted extremely thin, so I had to cut out a piece and weld it in.
This piece was a bit difficult to make as it has a lip on it, like the original floor, where it's spot welded. I had to make it from two pieces, the profiled top plate, then cut a small strip and form it to the top plate and weld those two together, if that makes any sense.

 

As I scraped away the seam sealer along the floor I found another area that was unacceptable, so I cut out a piece that took care of it all:

 

Now I was pretty tired of rust. I went over the whole rear, and found a couple of more small places that had to be  fixed, but as far as I can see, I got them all.




 

This is just what I have been doing the last days. the same place on the car except my car was not as rusted as yours, Mine is a 2005. Looks like it started in the same places. only my right corner needed weding (now).
I have an old Kemppi 250. should have bought a new handle and hose kit.
There is speciel drill bits for spot welds. I got two kinds but rarely use them.
Most often I grind and chop the rusty buit off, cut out more than I need to get rid of the laminated sheets of steel. I use a sharp chisel to hammer off spot welds too.
What are you coating it with outside and inside?
I have OK experience with FerroBet, Owatrol, Arcanol in many layers if there is no time for the good 2K stuff think I see you use. "gule gorba" some call it  (Norwegian brand names)
The thinner Owatrol gets inside and seems to do a good job in slowing down the rust combined with Arcanol. And Ferrobet seems to do good on rust. This based on cars I had for a long time.
Your welding looks good. Not too much heat into the next sheets it seems and strong. Mine do not look as good all the time, I find the correct setting on my welder when I am done I guess. 
 

Yakuza, I decided to coat the underside with Owatrol C.I.P. This was the only rust killing / stabilizing primer I could find that promised to be compatible with 2K paints. The downside is that it takes a long time to dry, I left it for 7 days before putting on epoxy primer. The yellow stuff is Hagmans CA 2K epoxy primer. I used a spray gun for everything. After I had painted everything yellow, somebody told me that yellow was probably not the best primer color for a black paint, as the yellow could sort of shine through. So then I painted over the yellow with a black epoxy primer. The underside was then coated with a 2K urethane top coat, and the outside of the car was sprayed with a spray can "matching" the original color, 0DG, Black Saturn Metallic. Then a layer of 2k clear.
The result was actually horrible, but I'm not that concerned. It's an old car, and rust free and technically sound is the important stuff.
Unfortunately, things tend to escalate. So I had no plan initially to do anything else than to sort out the rust, but since the axle and tank was already taken off, why not do so some about it I will post some pictures later.

Personally I don't like unfinished stories, so I'll tell you a bit more how this went.

The original problem is now taken care of and replaced with new parts. No more rusted body mounts:

 

It's back together and with a good backyard coat of paint. I did use 2K epoxy and a couple of cans of the original paint code, and 2K high gloss topcoat, perhaps a bit too glossy, but it doesn't matter.

 

As you can see here, I'm no pro, the transition is not good, but I might be able to smooth it a bit with some 2000 to 3000 emery paper and water, and then it might or perhaps not, "buff right out" as they say. It's not very obvious from a meter away, and it's an old car. I may just leave it, we'll see.

 

So with the painting done I decided to work on the tank. It was really bad. Very corroded around the seams. Initially I tried to sandblast it, but it didn't work very well. Whatever they had coated this with originally was very resistant, so it took 2 hours with stripping discs on the angle grinder to get most of it off. Really dirty job. Sandblasting took care of the rest, but also uncovered some holes around the smaller hose, I guess it's the vent line. At that time I more or less decided to go for a new one, but somewhat hard to get and expensive, at least to me. I googled fixing tanks and saw that soldering was an option, and since I had some thin copper plates in the garage I decided to give it a go since the tank was pristine, absolutely like new inside. That worked very well actually and I was really happy with the result. Hung it up with some gas in and no leaks after an hour. Replaced the fuel filter and pump also. I guess close to 25 years of service is enough. Also the straps that hold the fuel lines and the cable was gone, so I fastened some stainless bits on with epoxy and mounted some rubberized clamps on.

 

Wit the tank done I had a lot of other bits to do:

 

 

The rear fender was a real mess. Connectors more or less corroded off. Had to rewire most and source new connectors.

 

Spade connectors in an open connector. Seems pretty strange to me, but I filled them with dielectric grease.

 

The floor is now done and got a coat of special rust preventive paint. Also put on some sound insulation. Don't know if it helps that much, but it didn't take that long.

 

The seal was torn some places so I found a universal one that fit really nice.

 

The interior carpet was nasty. I just soaked it in car soap and pressure washed it. Worked quite well. Somewhat worn, but clean.

 

Decided to install some new speakers and radio. I read that 5-1/4" should fit in the back, but they did not. So had to make a couple of spacers that solved the problem. 4" fit well in the front. Also installed a Teyes CC3 2K headunit with a 9,5" touchscreen straight from AliExpress. Bought an adapter for the early Jimny, so it looks absolutely "OEM". Interesting stuff actually, came with Wifi, GPS antenna front and rear camera etc. Installed it and worked right away.

 

Had to do the needed things under the car. Put on a 2" Trailmaster kit, adjustable panhards, reinforced steering rods, did the calipers, all bushings, seals, bearings, new kingspins, drums, discs etc..etc. Also replaced the catalyst and the exhaust pipe just because I know a guy that ordered a set for his Toyota from Ebay and got a set for of all things, my Jimny! He was not happy and couldn't return it, so I got it cheap.

The front came with the patina of close to 25 years.

 

And after:

 

When I bought the car there was a box included with some new manual hubs. I put them on, but decided to give the old auto hubs a go.
So I took them off and gave the old ones some maintenance. Also blasted and painted all the lines for the vacuum system and replaced the rubbers.
The hubs clicks nicely.

 

Did the back.

 

She shall not rust again, so we invested in some Fluid Film:

 

Fluid Film grease on the outside and on all exposed surfaces under the car:

 

I used 1 kg of grease and about 9 liters of Fluid Film, sprayed it liberally absolutely all over, inside and out. Likely too much. She smells really bad, and will likely drip for years, but I don't think she will rust again.

 

I also did the plugs, all fluids, filters etc. Then the roadworthy test to get my plates back. Passed with no remarks.

And here she is on 215/70 R16 BFG's, Vitara rims. Not actually legal, so I've bought 5 new Dotz rims with TÛV in 6x15, and ordered new Toyo Open Country AT3's that will go on.

 

Another one from the side:

 

So that's it for now. Next up is to get her on the new wheels and then off to get everything approved.