Cutting a hole/ square for easier fuel pump replacement

 After much fiddling with reverse polarity, repeated impulses, rubber mallet knocking to the bottom of the tank etc I'm out of ideas about  resurrecting the fuel pump, so will have to replace it. Makes no sense whatsoever to me to remove the tank for this, so I intend to cut a "window" on the car floor.  But the gap between the tank and the car floor looks really small and clearly a lot of care is needed to avoid pinching the hoses etc.

 How would you go about cutting the car floor? Thanks a lot for any help

1mm slitting disc on a grinder 
Slide a thin piece of plywood/sheet steel 
Between tank and floor 
Or slitting disc in a dremel tool 
Also have a look and do a search in the search bar I believe other members have cut a flap/hole previous