The thing is that the backfire isn't a lean pop. It's a loud bang with 6" of flame shooting out. To me thats unburnt fuel getting into the header and burning.
The other thing is that its rhythmic. If I wasn't standing in front of it and checked the firing order maybe 5-6 times myself, I would bet it was a crossed plug wire, but its not.
Hal is bringing over a ready-to-run distributor today and they are gunna give it a go. If it still does it after that, I'm starting to wonder if the timing chain went bad.
Also, its a MSD 6a box, no rev chip in it.