Let me start by saying I'm totally new to tuning carb/distributor engines but I'm an engineer so I know enough to get myself into trouble.

Have a 350 in a 70 Chevelle that needed an in depth tuneup. Purchased and installed new Holley street avenger 650, Accel HEI distributor, plugs, wires, removed/replaced intake manifold, and removed/replaced headers.

After all the installs and timing, I'm having an issue tuning out a backfire from the exhaust. At idle, it runs fine. With no load at partial throttle, I get a backfire if I throttle it quickly. As soon as I put it into gear, I get constant backfires and I'm severely down on power to the point of being undriveable.

I timed it to 10" base (vacuum disconnected and plugged). I was told to dial in total timing at 36-38". If I rev it up to 3k with no vacuum, my timing light is showing 44". This doesn't make sense to me because the springs installed in the distributor show in the manual they allow ~21" so wouldn't my total advance be around 10+21=31? With vacuum connected, I'm seeing about 47". A friend suggested I had too much advance and I should get a spring and weight kit. I did and put in a tighter spring that showed in the manual about 16". With that installed, my base timing was way off, so i reset it to 10" now without vacuum I was seeing a total of 25" at 3k rpm (still with backfire). With the vacuum reconnected I was seeing 50". I also tried turning down the vacuum advance by adjusting the diaphragm on the distributor but I saw literally no effect on the overall timing at 3k rpm.

I've double checked for vacuum leaks, exhaust leaks, sticking valves, improperly adjusted rockers, basically anything mechanical and can't find anything.

My questions are: with the backfire, which direction should i be trying to adjust: more or less advance?, Is there anything mechanical not listed that I could try that could be causing this (considering it had no backfires before the tuneup)?, and when you tune for total advance on throttle, do you leave the vacuum disconnected and plugged?

Thanks for any help. As I said I know enough to be dangerous to myself.