On some of the 1970's trucks there was a sticker on the door jamb with the VIN number. Of course the actual VIN number was on the door as previously noted. I have no idea what years / models had the sticker on the jamb. Over time, most of those stickers have probably faded and peeled off.

On the differences between cabs, some trucks had the fuel tank in the cab. The early 1970's models had a different dash than the later 70's trucks. I think there were a few very minor changes to the fire wall when the dash was changed...but I may not be remembering that correctly. Someone told me the 1972's had an different sized window. (rear window or windshield??) Some of the cabs had a removable cover to access the transmission.

On the legality of the swap, I'd take "Poorboy's" advice and check with your local government officials. There are other states which require a new VIN number for a cab swap. It may not be a big deal to get the new VIN number...but I'd check with the state first. Then you can make a more informed decision.