[66Submarine]

Should be a 4.57 I think, 32 teeth on the ring gear and 7 on the pinion.

I have the same rear in my C30, although in the 7,200lb rated version. They were made by Eaton from 1947-1972 and are known as the HO52 and HO72, yours being the 5,200lb 3/4 ton version. Gears interchange between both ratings, and they are a third member type, meaning if you locate another third member you want to use you can just swap it in and not mess with setting anything up.

A 4.10 gear was made which would make your engine speed 90% of what it is now and the third member will bolt right in place of the 4.57's with no mods. A T-5 5 speed is a common overdrive manual swap, and while I personally hate automagic/tragics in a driver, a 700r4 is another option.

My 65 C30's HO72 is something like 65" WMS-WMS, a later dually 14 bolt rear is 63.5" WMS-WMS (I think) and 3.73's are common in those.

Hope this helps,
Chaz

EDIT: Forgot you have a C20 with the coil springs and trailing arm setup, 14 bolt will require some mods somewhere to work. Also, not sure but driveshaft may not be the same length for the HO72 and HO52 third members, but 4.10 gears will probably be in a HO52 that came in an auto 3/4 ton anyway so you should be good.

Also, the C20 frame (I think) is the same as a C10, so you could also look at swapping to half ton running gear, seeing how you aren't really going to use it as a truck.