Let me add a couple of things here about sheet metal and "hard" parts.
The dashes swap to BOLT IN but you the separate wings, uprights, and windshield pieces are particular to the model year. You will also have differences among models as a hardtop has a lower windshield, the Sunliner has its own pieces of trim, etc.
The steering column in a 56 is shorter than a 55. The angle is the same, the column OD is the same and all, but you will need the pieces that give you the shift indicator, the tubes for the turn signal wiring, etc. Underneath, the pedal arrangement or measure is the same, but there is a slight upgrade to the 56 model for the pedal bracket - especially if you have a standard transmission.
More fellas may chime in later on this.