D -- Jim is correct -- Cabs interchange with the pickups, just be aware that the cab mounts are different '49 and later. Also, in '49, the gas tank came inside the cab so as has been observed in a different thread, if you use a later cab on a '48 chassis, you could have two fuel tanks
On the engines, after '53 ('54 and up), the I-6 engines (235's, 261's, etc) have longer water pumps (and mounted lower). If you use a later engine, you will have to modify it to use an earlier water pump (not hard) or move the radiator forward. You might also have cooling issues, depending on how you use your truck.
For other differences, check out the Advance Design spotting guide in the Tech Tips (Menu is at the top of the page).
Good luck,
John
