The 4-digit key code should be located on the lower portion of the door handle body. So, if your locking door handle is still in place on the truck and you can remove it, even if the lock cylinder is missing, you should be able to see the key code and order a replacement key. If your truck has the original ignition switch and door handle, the ignition, glove box lock, and door handle should all be keyed the same. It may be a long shot but worth a try. If you do a search on eBay, you can find some 1939 GMC/Chevy locking door handles for sale and a few have pics that show the location of the key code.
My ‘39 Chevy was still set up with an electrolock but I wanted to switch it over to a newer coil setup. Because I had the original key, I was able to remove the ignition switch from the electrolock ignition switch body and purchase a vintage, but newer, 3-post Delco 1116507 ignition switch body that my ‘39 ignition switch fit into and that accepted the original ‘39 bezel. Looks identical on the dash and works great! I’m not sure if the 1116507 configuration is compatible with your GMC’s original ignition but it worked on my Chevy. Good luck!
