What Differential is a good replacement?
 

My 1960 K1000 has a bad rear diff. Would like to find a used one. Rebuild is too expensive. It's now a 10 bolt, believe to be HO35. Not many around with 3.92 ratio. Would like a 12 bolt. Also what about a 3.73 rear, they are more available. Is this too much mismatch?

Also a few available 55-59 rears with 3.90. The covers are WWI helmet shape, mine has a ring gear bulge shape? I believe HO35 was 55-62....so were there different covers or do I not have a HO35?

Re: What Differential is a good replacement?
 

Generally half ton GMC trucks from 60-66 and later years up to 68 came with the Dana 44 rear differential in both 2wd and 4wd. At some point the one in your truck was changed out. A 12 bolt would most likely be more readily available but the correct differential would be a Dana 44, same goes for 1955-59 GMC trucks.

Re: What Differential is a good replacement?
 

My 1960 GMC Manual says the available Diff was HO33 front and HO35 rear. My cover looks like a Dana 44.