Have you looked at Doug Thorley? They have a pipe that bolts on for the cross over, And from what I have seen they offer headers for lifted trucks, I looked at their site and didn't see much for the 67/72 stuff but I could of missed it.
Doug Thorley
I have been told and heard that a single 3" will out flow two 2.5" and before I sold my 83K5 I had planned to do pretty much what you have in the works.
Ceth