All the steel 4.0 gas tanks and pumps are interchangeable, but some earlier ones, though they are the same physical size, do not hold the full 20 gallons. Make sure if you can that you get a proper 20 gallon one. Unfortunately, I don't know how to identify one from the other, but I'd opt for the most deluxe model of donor, all else being equal. If you're looking at used, look very very carefully at the tank, and make sure there are no rust holes developing on the top, and that there's no seepage at the seam. It may depend on where you live, but corrosive crud can come to rest on the top and eat through it, and though you'll never see the leak you'll smell it. Take one with top rust only if it's free (you can epoxy the holes). Don't bother with any tank that is getting damp around the seam. You can't seal that kind of leak.