Fuel cell mod

[tregrad]

I have an after market fuel cell in my 72 Blazer, I bought it with it in it. When ever i need fuel I have to climb into the back, flip a hatch door open and fill it that way, total PITA for an old dude like me.
Well today i dropped the tank because im installing a new hitch receiver (Uhaul part #75914 and its burley). and want to weld a filler neck onto and run the hose up to the original neck so i don't have to climb in the bed to fill it every time. since i have it dropped I figured I might as well make sure the pump and everything looks good etc. Well when i pulled the pump and peered into the tank there is all this red s#it everywhere what seems like a sprayed in liner, not really a bladder so-to-speak but more of a liner and its coming loose on the sides and im sure its going to be harboring gas in between the folds where its peeling away from the walls.

Question:
1. How do i do this safely? My thinking is to fill it with water to the level where i want the filler welded on or even higher, drill the hole and gnarl it out to shape i want, then lower the level a bit and weld the filler neck pipe on?

2. That said, I think there will be a lot of water left in the tank between the fold where its peeling after i drain the water out, how do you dry the water out?