IIRC, '65 half tons came with a 3.73, with a 4.11 and a 3.07 optional, and the 4.11 and 3.73 could also be had as positraction units.
1st gear in your tranny is granny gear and 4th is direct drive (1:1). My truck has a 3 speed and a 3.73 rear in it. It does 55 easy and can maintain 65 on the interstate although it gets rather loud in the cab from the 250 roaring. Any faster than that and you're beginning to push the engine.
