you gotta z it at least 3.5 inches, or, use a crossmember kit built by porterbuilt hot rods. It's called a dropmember. Its a bolt in crossmember that allows you to lay level with full frame. It also converts your steering over to rack n pinion and narows the trackwidth for anything over a 22. heres a link to a thread in the 67-72 forum...
http://67-72chevytrucks.com/vboard/showthread.php?t=201898