Measurements are the easy part, the hard part is finding the '39 rear fenders! I believe about half the "1939" rear fenders I see on ebay are mislabeled '40-'46 rear fenders. This includes the fender currently on there. I would have some questions as to the width of the fender if I was interested in it. It looks narrow to me, but it's hard to tell from pics sometimes.

To be long remembered as he's cruising in
The Passing Lane