46-72 eaton rear ends have a lot of interchangeable parts. I believe you will find a 4.11 pumpkin that will interchange in 67-72 3/4 tons that have coil spring rear ends. I have never done this swap, and skimmed some other 3/4 ton rear end threads to come up with that answer. 3.90s seem to be very rare. The late Art Hauser had offered them. I'm not sure where you are going with your truck, but 4.11 gears and an overdrive would work pretty well to move your truck down the road. Taller tires could make up the difference between 3.90 and 4.11
