My 340 in my 66 developed issues :-(

I can't help you with the header ordeal. I mess mainly with 67-76 A-bodies.But I want to ask, did you verify that it is a MP 284/484 cam? If it is I was going to suggest you install it @ 104 Center Line. That is where that cam always worked the best for me and was advised to do this over 20-25 years ago by a MP tech guy I was camping next to at the Gator nationals.