just to follow up, here is a dynacorn page showing the 54-55.1 are 1" shorter than 47-53.
http://dynacorndepot.com/running-boa...s-chevy-pickup
and here is the picture I have of a 54 truck with a 47-53 apron.
this might be helpful because shows a couple things, first it shows how I just stuck the extra under the bed after bending it, second, this truck was a 54 with a 47-53 bed on it before I changed it to a 54-55.1 bed, so you can actually see the pattern from the 47-53 apron on the front edge of the fender and how the 54-55.1 apron is shorter.
hope you get it figured out!