I've got a '73 Barth with the 454. It's on a 1T box van chassis. I might be able to help you out and shoot you some pics if I had an idea what you were looking for. It does look like I missed your deadline though. Let me know if you want the pics. Good luck!
I would look at an exhaust leak. Even with TBI, most SBC engines have a bit of unburnt fuel present in the exhaust. If the exhaust pops especially on deceleration, you are getting cold air into the exhaust stream and it may reignite the unspent fuel. The popping would be close to the headers or...