My daughter has a 1976 F250 Camper Special, it was involved in a wreck and has a swapped bed off 1974 F250 and also has dual tanks and both tanks are brand new. The problem we are having is that the fuel is coming back up the filler neck and leaking out of the cap onto the side of her truck and the ground. We have a locking cap, but its older and kinda loose fitting. Do you think we should replace the fuel cap to fix this issue?

 

Swap the two caps side to side and see if there is change.

 

Some trucks with thermactor used different caps and necks, perhaps, the new box has filler necks not compatible with the old. If you still have the old one, try swapping the fillers back. The Ford type locking cap should click twice as you push it down, if it only clicks once, it is not going all the way down. This would also indicate an incorrect filler neck.