need help picking a new rear end

[Se7en Deuce]

Ive tried searching but im not coming up with answers a noob can undestand

I have a 72 c20 LWB 3/4 ton 8 lug pickup with coil springs
(hope that was enough info ^ )

truck was pretty well maintained and runs excellent, so I figured the 1st mod I wanna do is to lower it.

Found a kit for 1700, but this will change my bolt pattern in front to a 5 lug, naturally I want the rear to be the same. Im hoping to find a used 5 lug rear end locally.
My questions is: what will fit, should I look for one with Posi Traction and what gear ratio should i look for, I intend to build up motor at a later time.

sorry for the noob questions but Ive searched alot today and just didnt see any straight foward answers.

thanks for any help