I went thru all of this on my 54 build before I decided to go with a 78 F150 4x4 chassis. I mocked up my 54 body on a 84 F250 4x4 chassis. I'm building my 54 F100 to be a year round daily driver/work truck.

The 80+ trucks have the engine set back 8" farther then the 56 does. So it requires a very large firewall recess.

The cab will need to mounted with at least a 4" body lift. The front inner fenders will need to be trimmed to fit. So will the front frame horns ( hit air deflectors & valance ) .

And if your 56 has the small wheel well openings you might want to consider the F250/F350/F500/F600 large wheel well opening front fenders. They are a hair wider and fit the 80's chassis track width great. The larger wheel well openings also allow you to mount the cab around 1.5" farther back too. This means a slightly smaller firewall recess. That's what I'm using on mine.

Check out my build thread there are a lot of pictures. Someday I might actually finish it.

https://www.ford-trucks.com/forums/7...-build-11.html

 

How about fatter front fenders for a 56 f250 going on a 2011 e550 18,000 gvw. Is there a mfg of fenders glass or steel that will make each them 8 inches fatter? Getting a 21 ft rollback carrier on back is why we need the 550.