A '55-'62 1/2 ton center section will bolt into your housing.
That will get you open drive line with 3.90 gears.
A T5 overdrive 5 speed transmission out of an S10 can be used with minor mods. Both of these swaps I believe are in the tech tips section here.
You will also need to have a driveshaft made, or use a junkyard driveshaft if you can find one the right length.