I know that deal with only a 26 gallon tank! My old 97 F150 with the 5.4 had a 26 gallon tank... It got to 1/4 tank so fast!
Got the 13 F150 with the 36 gallon tank and where I was stopping for gas on a 200 mile trip, I don't have to worry about gas at all until I get there.. I still stop every couple of hours just to stretch the legs and such, but not having to get gas makes it an easier stop..
As was said, if you can get a replacement tank with the extra capacity in the stock location, that's the way to go...
Good luck!
Mitch