What wojo said! OMG!!!

 

that has most definitely never ever been a thing.

every different wheelbase has a different frame.

 

Not necessarily, a regular cab 8' box might be the same wheelbase as an extended cab 6' box.
A reg cab 6' box from the 80-96 models were the same wheelbase as a bronco or only off by an inch.

EDIT:
Perfect example, Reg cabs with 8' boxes, super cabs with 6.5' boxes and crews with 5.5' boxes all have 144" wheelbases. Why would ford make 3 different frames? They'd make one frame and have it compatible for all three configurements. I'm sure if you crawled under those 3 different trucks, from the front doors back mounts would be different but from the B pillar forward, they might be the same.

 

okay. so yes on 04+ F150s, the regular cab/8' bed, supercab/6.5' bed and supercrew/5.5' bed share the 145" wheelbase. my truck, a supercrew/6.5' bed is a 157" wheelbase. a supercab/8' bed truck has a 163" wheelbase. the rare supercab/5.5' beds are 133", and regular cab/6.5' beds are 126".

but regardless, this is all completely irrelevant to what the OP is asking unless he's planning on changing his cab as well, or cutting a foot out of his frame.

 

Not arguing that. A 5.5 bed on a frame built for a 6.5 would look goofy with the gap.