There are differences in the gravel pans and air deflectors over the different years.
The '56 air deflectors have depressions for the headlight buckets due to the recessed headlights.
The '53 uses one gravel pan, the '54 uses a different one (due to the shape of the grille) , and the '55 and '56 use a different one. It has to do with the running lights and their position.
You can use the '55 grille on the '56 truck without any problem. If you want to use the '56 headlight rims, you will have to use the '56 air deflectors and will have to "French" (recess) the headlight buckets. I've seen it done but I don't know the details.
Take a look at this thread http://www.ford-trucks.com/forums/10...eadlights.html, I believe that all your questions will be answered there. Post #65 has some good info on the bolt patterns, that was a gotcha I missed when I decided to go with a 53 grille in my 56. Going from a 56 to a 55 may not have the same bolt pattern issues but it is worth while to look before you leap.
I noticed on my truck that the bolt patterns aren't the same for 55 to 56. The po had one deflector from a 56 and one that was original. When I put my 55 grille on the for test fitment, the 56 deflector wouldn't bolt up bc the bolt holes were in different spots. I don't know if mine are made different or what but that's how mine is.
I knew there was a difference between the '53 and '56 grille-to-deflector bolt pattern, but I didn't know about the '55. It's easy enough to drill the correct holes, but be sure to plug the extras. Otherwise the front tire will be throwing water, mud, and grit behind the headlight bucket. A sure recipe for rust.