73 and up 12 bolt into 72?

[muddpile]

Greetings! I have a 12 bolt of unknown origin in my 72 C10 LWB leaf truck right now, but unfortunately it has seen better days and needs to come out sometime soon. I have a 12 bolt I saved out of my 1976 (I think) pickup that I sent to the crusher. It is in much better shape if I recall, but I'd still like to do some work before I put it in.

-Will it fit my truck? I have a flip kit, so the spring pads will need modifying anyways, but is the width okay generally between these two body styles?
-I'll be doing a rear disc brake swap on it, any recommended kits? I've looked closely at CPP.
-Is there a basic freshening up kit with bearings and seals and whatnot that I can purchase for peace of mind?
-I'd like to swap in 3.73 gears (I believe it has numerically lower at the moment) but I don't know what's involved, as I've heard there is a 3 and 4 series carrier, and don't know which I have.
-I've always used just plain old 80w90 gear oil. Is that the best stuff for the job.

I will not be doing the gears myself, I have a guy for that, I just want to make sure I'm not forgetting something when I take it to him.


Thanks!