Just drove about 150 miles, and the last 30 it wanted to die below 12-1300rpms. Also if I give it a little extra gas, it will backfire through the front barrels. (edelbrock 750)
I felt a little crimp in the gas line, so I fixed it. Still the same problem. Will not idle.
When I get it started and rev it with the air cleaner off I can see it backfiring on the upside of the revs, it doesn't when backing off.
I will check the hei next. I am just hoping it's not cam/valve related. I have been babying it as it should be just now broke in (about 500 miles).

ANYTIME your backfire throught the carb. it's to do with the valve/ignition timing.

Perhaps the distributor just slipped I have had dist hold down bolt come loose and dist turn on its own just a thought

Bad Rat, thanks, but that is not always the case. If the carb is too lean it will also cause a backfire.

Check your cap and rotor. I had a 81 T/A with a 305 chevy motor and it started acting funny, missing and backfiring, turns out the rotor was cracked
-Bill
'66 SS 396 in pieces

The very first easy item to check out is the ignition timing. Next, carb or valve lash, take your pick. Adjust Idle mixture screws to smoothest and highest RPM, check your jetting,
set your valve lash, check distributor cap and rotor. BTW, I'm a firm believer in checking valve lash because I've seen various problems related to
incorrect lash. Let's also assume that you don't have a flat cam although all your symptoms don't
point to that.

I think a wiped exhaust lobe, stuck or bent intake valve, and a broken intake valve spring can also cause a backfire through the carb.

I hope it's in the ignition.