Some careful drilling with a step-bit and an air-powered nibbler or a well-wielded 4" grinder with cutting wheel would make quick work of the floor steel with very little risk to the tank or fuel.
The video I mentioned earlier (and a couple like it) show some interesting ways to create a new flange with sheet metal strips and caulking and blind rivets. Resulting in a flange for re-attaching the cut-out floor section. And you can do that with short self-tapping sheet-metal screws.
My intent is to do that work in advance of need. As Stryder is alluding to, the idea being to make a 'surprise!' fuel pump swap in the field easier to do. Maybe an hour instead of half a day, in the middle of an 'expedition'. Would sure beat hell out of crawling around on the ground, away from home.
This is the specific video that lays it out for GMT800 Subs, even gives dimensions / cut placements. Makes it look pretty easy, to me.
The center of the pump appears to be right on the center of the seat bolt pattern. The hatch hole is 2" larger to the front and centerline than the other two sides, apparently for greater access to the plumbing connections.
Sure seems a lot easier than trying to wrestle the tank out in the driveway. I've watched some 'pro' fuel pump swaps and even standing up under a lift it looks like a great pain tring to tip the tank and get your arm up on top of it to disconnect things. There isn't enough slack in anything to get the tank loose and put it all on the ground first. Looks SO much easier to just reach thru a hole cut in the floor. I've wrestled around under vehicles enough in my 54yrs to know I don't like it. Grit in my eyes, contortions to reach things and needing a second elbow in your arm. Losing lots of skin. No thanks. I've got the tools and metalworking skills to make short work of cutting the floor and then building it back up.
Some good flange and caulking work underneath, some peel and stick butyl rubber window flashing to seal the floor on the interior, would be good enough