Not to hi-jack but this is EXACTLY what I'm up to. I found a '72 C30 truck in a local yard today. The stamping says RAK01122 on the rear diff. Guy says that makes it an open drive rear for '72.
Is THIS the magical one I need? I don't care if it is an open diff or not, I'm not pulling the house over. Just trying to get my 3804 "Ivan" to move a little better.
