I did some work on a 57 truck a while back that had a 65 Mustang tank. It fits in between the rear frame rails. The bumber brackets have to come out to get it in. The filler is straight up through the bed wood.
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