Consider a Mustang tank ('65-73). There have been tech articles about using them; the top is flat, actually the trunk floor in the original car. Later ones are larger, I think, maybe as much as twenty or more gallons. Available brand new from suppliers of Mustang restoration stuff.
You could try looking over at www.fordtrucks.com They have had a couple of write ups on the subject. What model of truck is it? I have a couple of unibodys I have been keeping around for a couple of years
You might see about an "E" body Barracuda tank, or a '52-4 Ford passenger car tank. Either of those could possibly be the right shape, and both are rear fill, so you should be able to have a gas cap accessible underneath the tailgate, as opposed to having to cut a hole thru the bed floor, or side of the quarterpanel. The Ford piece has a filler neck on it which may or may not be long enough. -Bill
I second the Mustang tank! Although ten years apart i put one in my 54, Slid right in between the rear frame rails.
The early Mustang works well for a rear fill, a Bronco II is a popular swap for a side fill. www.fordtruk.com is 1961 to 1966 only Ford trucks and has plenty of info about gas tank swaps.