The Mustang tank will fit between the frame rails in the rear with the fill neck through the bed. There are numerous aftermarket aluminum tanks available as well.
The spare tire will have to be mounted somewhere else, however.

Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine