The Stovebolt Site Forums The Shop Area Driveline '51 3600 3/4 ton rearend swap

The Eaton HO-52 / HO-72 like your truck has was used 1947-72. 4.10:1 gears seem to be a lot more common than people think, but that is the tallest they came. Very easy swap (as mentioned) if 4.10:1 works for you (I have 4.10's in my truck). There were some aftermarket 3.90 gears made recently that were expensive and are now pretty much non-existent, as far as finding them goes. (IIRC, I think some claim that 3.90's were made by GM at some point, but I have never heard of anyone ever finding them.)

I'm by no means sure, but a later (73 and up) 14 bolt out of a cab and chassis (narrow) dually may work; some of them were 3.73:1 (BBC powered trucks, IIRC).