which diffs on 60-66 C10s?

[Brad54]

The GMC trucks, at least from '60-'62, came with a Dana 44 rear end, and that was an optional rear for the Chevy trucks as well.
I've got a Dana under my '61 Suburban.
I'd have to go back through my notes, but I BELIEVE the GMCs were leaf sprung, while the Chevy was coil/trailing arms... but the HD version of the Chevy was available with leaf springs, and the HD version for GMC was heavy coils and trailing arms.
The Dana has a rear cover somewhat like the later 12-bolt... it's kind of Home Plate shaped, but has 10 bolts.
The Dana 44 is the most common rear end in the US, used back in the '50s on Hudsons and Studebakers, and up through the present in Jeeps, Ford Exlorers, and a version of it is even used on the Viper and the last version of the Corvette (with the trans up front behind the engine). Parts and rebuild kits are readily available, brand new OEM gears from Dana are still available (better quality than aftermarket gear sets), and you can get a brand new Posi unit from a Jeep guy who is swapping his 3-series gears for a 4-series. (Got mine for $100).

-Brad