What is your gap between the cab and bed? Recent discussion on here suggests that it could be anywhere from 1/2" to 2" or even more. I think it should be about 1" at the center of the bed, but I can't prove it. If there is no spare space there you might have to do some refab work somewhere. Loosen up the running boards and see if you can get a little bit there and you might be able to loosen the fender bolts and get a little movement there too. I suggest get everything loose and see if you can tweak a little here and a little there.

1953 Chevrolet 3600
Fried Green T'mater (Vern)
In the Stovebolt Gallery
Caution: Political Statement
Man who runs in front of car gets tired, man who runs behind car gets exhausted.