Well, I was sort of successful
Turns out the stock NOS “75-80” downpipe I was sent had O2 bungs and a square convertor flange. Sooooo, I had to go with what I have instead for my 3 month re-tighten job. I had the 3 month exhaust leak growing and had to do something so I could do the annual emissions check. So this weekend was it. (In Albuquerque any vehicle ’75 or newer gets emissions check. It is not a rolling date either.) Maybe in another 3 months I will have another go at it with a right and proper down pipe. Who knows.