I think it is valve train or electrical:
- check for broken v/springs
- what ign system? If dist is electronic & has a magnetic p/up, check the air gap. As rpm increases, so does centri force. If the p/up air gap is too tight, it could make contact at higher rpms with the reluctor & cause a backfire.
The engine will also run MUCH better with about 20* initial timing...smoother idle..... more vacuum.