First, congrats! I have one on the way as well (May) but I was lucky and got my truck done first.
I had an Ho52 Rear in my K20 and looked around for a dana 60 out of a GMC from the same year. I found one out of a 71 K20 and it dropped right in with no mods at all. I did add rear disc brakes, but spring perches were perfect as were the shock locations. There may be Dana 60's out of some fords that work as well, but the GMC's had Dana 60's while the Chevy's got Ho52's for some reason.
At least that is what I had read here.
Good luck
Chris