Hy The 38 Special, as far as I know the only difference between the Chev. and GMC frames you're asking about, is the front crossmember. In 1939 GMC started to use a new engine, their own manufacture, it featured full pressure lubrication, and was a little longer than the Chevy engine of the same era. My parts books don't list complete frames by part number, or by side rails, the books say to order those parts using serial number, so I can't confirm if there are other differences, hope that helps.
