Backfiring, why?

[FF33rod]

Okay, so managed to have a first start a week or so ago but didn't have the cooling system finished off so I couldn't run it much at all. So now have that in place and put the mufflers on it so that I could run it in for 20 minutes or so. Gotta say, not a whole lotta difference adding the mufflers! But the problem I have is the engine starts fine and then after a few second I start here "pops", it starts backfiring. Seems to get worse the longer I let it run. I double checked distributor relative to TDC, seems to be close. Then as it was idling I moved the distributor around a little to see if it would help, it didn't. What now?

Setup - Ford 347, short block with Trickflow Stage 2 cam installed from TRE performance, AFR 195cc heads, Edelbrock RPM Air gap manifold, Holley Sniper and Holley Hyperspark distributor and ignition....

This is a brand new engine, so I do not have an old standard ignition system to put on it to start off. Have to run with the Sniper controlling the timing. Followed the setup procedure for the Sniper - running the setup wizard sets displacement, # cylinders, etc. As I said, starts fine but then.....

Is there any chance the dizzy is 180 degrees out of phase? I have zero experience in building engines but I thought that if it's 180 out then it wouldn't even start. Not sure what else to try though....

Suggestions please. Thanks guys
Steve

Edit: note that a video has been added, post 24 below which demonstrates the "popping" issue much better than can be described in a post