rear ends- what years are a bolt in?

[mrh]

Hello all
this is my first post. I just picked up a 67 gmc stepside project. its for my stepson and is going to be a father/son project. it is obvious to me someone has put another rear in the truck since both factory shock mounts on the truck are forward of the rear and the rear in it now has staggerd shocks. someone just drilled a hole in the frame to mount the rear facing shock. My question is what years will just drop in? it is the factory leaf spring set up. Here is a pic of her. Thanks in advance.