Ball joints and tierod ends. If your truck already has disc (I'm assuming 5 lug?) it either has the 71-72 style or the 73-87 style. If you have the 73-87 drop spindle and it DOESN'T work with what you have then you probably need the ball joints for a 71-72 and the right tierod end for the 71-72.
That being said, people do some crazy things to these trucks... sometimes it's hard to figure out what has been done.
Have you checked this link?
http://67-72chevytrucks.com/vboard/s...d.php?t=444823
Good luck, Rg