Hey John,
I have a 39 GMC and may be able to answer some of your questions.
Cab are the basically the same from 39 thru 46, but if you look at the cowl you will see an X on the later cabs, I guess for strength.
Windshield is the same from 39-46, rear window however, is the same from 39-42 with the die-cast frame.
Side glass is different in the earlier models. In 39, GMC used a window frame (chevy did not) and so it best to have it cut locally.
Doors will fit.
Dashboard was different in the 39, along with the window crank. 40 on up is easier to obtain parts.
Since the 228 CID engine was longer than the Chevys, the front where the headligts mount are not the same. Only 39 and 40 GMC's are the same.
Headlight buckets are not the same, 40 used sealed beams. Mounts are the same.
39 & 40 Grill are the same. 41 on up is different and so are the front fenders, hood and running boards.
The 39 GMC had the same size bed as the 40 chev. So the bed with is the same except for the 39 chevrolet. GMC script on the tailgate.
I dont know of any metal repo fenders. Front fenders are hard to find. Grills are very rare and no GMC after market grills.
Park lights wer mounted on top of front fenders 0n 1940 Chevys and GMCs.
There are fiberglass floors available. I fabricated my own from a metal eletrical breaker panel.
Good luck in your search for parts. Hope this helped, I'm sure other bolters will have more information to pass along.
JIM
