if it's a ford frame, the tank out of any ford van/truck should fit. You just need one of the proper vintage. I know the older trucks with dual tanks had 1 tank that ran down the drivers side frame rail from the tranny to the rear axle and the 2nd tank ran across the entire back from the axle to the bumper.
shoot you've done the hard part and figured out which tank it is. who knows, you might get lucky searching for a gas tank on e-bay!
I know I ran across a couple of companies that sold larger tanks for my dodge in that 35-50 gal size.
try googling "ford replacement gas tank"
I've dealt with this company before, good car parts source:
Ford Fuel Tank - Ford Gas Tank | Free Shipping