It's a Eaton rear end. A pumpkin from a 68-72 3/4 ton should swap out directly.
You could find a 4.10 or more likely a 4.76, 3.90 will be near impossible to find.
I've got a 46 DS pickup, where you at in California? You could check out my T5 behind a 235. I decided to keep the 5.14 rear end and use a .63 o/d. Works out to be a nice combo.
Greg
