How difficult would it be to put a 1955 cab onto a 1956 frame? I know there are slight style differences between the two but is there a mounting difference?

 

No cab mounting differences. To my recollection the only differences are the access panels in the floor to the M/C and battery. Slant cabs had a metal plate over the M/C and the 56 has a rubber plug. The slant cabs had a battery that was more square whereas the 56 had a long narrow rectangular battery which made the access cover a different shape. The running board mounts on the driver's side on the 56 may or may not have the proper mountings for the saddle gas tank.