Hi stea
From everything I've read the 39 Chev fenders were wider than 40-46 because the bed was narrower for 39. My 39 GMC however, has the same size fenders as my 46 Chev. I've also wondered about the rear fender width on the 39 GMC. It may be as Dale suggests, GMC was ahead of Chevy on increasing the bed width.
GMC More Power More Payload :p
