tryin to break in new engine, ruff idle

I shut it off after 5 min at 2100 rpm because I didn't like how it kept missing every 5 seconds and it would drop 2-300 rpm, sounding almost like muffled backfire.

I checked the gap on the magnetic pickup of my new distributor and it was .009 to .010. I converted to electronic ignition with the new motor.

1976 LA 360
punched 30 over, stock pistons and rings
fairly big summit cam, probly too big
edelbrock torker340 intake, holley 600 carb, rebuilt when?
stock 360 heads rebuilt w/ new stainless valves and double springs for big cam
headers
elec ignition w/ orange box

What do you think the top one or two likely culprits might be?

Thanks for your suggestions