If you have the ability to install motors and transmissions by making mounts, you have the ability to modify a couple of frame crossmembers to accept nearly any gas tank you want to install in your truck. Its not really that hard.

The limiting factors are the length of the tank, it pretty well needs to fit between the rear axle and the rear bumper. The other limiting factor is it really needs to fit side to side between the frame rails, though exceptions can be made if you know how to weld. The end height of the tank should also be a consideration. It would suck to mount a tank that is tight up against the box floor, and is still laying on the ground. The last thing that may cause issues is how the tank needs to be filled, and where the sending unit/gas outlet, ends up.

I have installed a gas tank from a a late 70s Cordoba into the frame of a 54 Dodge pickup. That tank was 25 gal. I believe I modified one of the crossmembers slightly. We cut in 1/2 a 40 gal Bronco tank and mounted it into a 47 Plymouth. (It was too high in original form.)
I've installed a 66 gal bus tank into a 73 Dodge pickup, that one rode in the box (it was the 3rd gas tank on the truck, had the 26 gal saddle tank, the 20 gal rear tank, and the 66 gal in the box, 112 gal! Couldn't afford to fill them all if they were empty, but at 8 mpg, I still had a long range before I had to buy gas).

Bottom line, measure the tank, and measure where you want to put it. See what will need to be done to get it there. Gene