Re: new engine popping/backfiring under load
Yes, it could be valve train, flat cam, valve spring, cam in wrong etc. etc --
Before assuming the worst I'd start with the simple and cheap - it's far more likely.
Try it without vacuum advance connected - see if it's related.
Check timing advance -
no vacuum, idle, S/B around 8.
No vacuum 2500 - S/B around 20-22,
vacumm connected idle around 8 (same),
2500 around 35.
Use an advance timing light or mark new marks on the damper -- just use the 0-10 degree marks as the width and extrapolate.
Check exhaust for plugged. Vacuum gauge on intake manifold vacuum - idle around 16-18, 2500 about the same as at idle.
Snap throttle hard - should drop to near 0, snap it shut fast - should bounce up higher than before.
If no to the above, suspect exhaust.
Check coil wire - easy way replace with a known good one.
Make sure plug wires are NOT running parallel for much distance, ugly runs nice, pretty doesn't.
Check these and let us know.