Are you talking front or rear fenders?
The front fenders should be basically the same for a 39 / 40 half and three quarter ton truck with the exception of the parking lamps on the 40. The rear fenders are different however with the 40's being 1-5/8" narrower than the '39 due to the width of the bed. I've seen a few big bolt 39's and 40's and they have pretty large tires on them.
I've got 7.5x16's on mine and that is about the tallest tire I'd be able to get under my fenders.
The 39 and 40 trucks are basically 'tweener trucks because the 39 has a lot in common with the 38 while the 40 has a lot in common with the 41-47 first series Art Deco trucks. These are my favorite bolts and while they appear nearly identical there are a lot of subtle differences between them!
Someday I want one of each.
