69 Custom/10 1/2 Ton rearends???

[Longhorn Man]

just about every C/10 ended up with a 12 bolt in it. A handfull ended up with a Dana, more GMCs than Chevy trucks did though... but still a rare find (and I hate useing that word)
As far as gearing, they came with 4.10, 3.73, and 3.07. They were all standard with certian engine/tanny options.
General rule of thumb without breaking out my books...
3 speed on the column got 3.73.
4 speeds generally got 3.07 (possibly not the case on inlines, can't quite remember)
Anything with a factory overdrive (rare and weak unit, only on 250 and 283/307 motors) got the 4.10 (possibly 4.11)
small block with auto got a 3.07
I *think* big block with a 3 speed got 3.73... they only built a few. I know 3,07 came with the automatic with big block.

This is off my memory... but I think it is pretty close. A few years worked things a little odd, but these are the generic later year guidlines.
Also, there was then optional gearing. The same nubers as above, but for example if you wanted a small block with a 3.73 and an automatic...

There was talk a few years ago of some GMC trucks with 3.54 gearsets in a dana rear. I have never seen documentation of these in factory lit... but it seems they made a few. Consider this the exception, not the rule. If memory serves, they were in inline powered trucks.