I did not read all of the pages, but from what I got out of page 1, I think you should give strong cosiderations to find an old RC Engineering (Russ Collins) header and muffler. That is THE best sounding exhaust on a CB750 bar none AND has the approz physical dimentions you requested. I have one. It is NOT for sale, and I don't have any pics of it right now. I will have to look inot my old RC literature, but I think it was called something like a Scavenger or something.