Good questions. A couple things come to mind.
1. Make sure your timing marks are accurate and TDC matches your mark on the harmonic balancer.
2. Sometimes pinging can be caused by overheating. Is your car running at around 190F, verify, don't trust the gage. Does it get louder (pinging) as it warms up?
3. Does your timing repeat and not adjust itself/wander when rechecking after a few days? I know your engine is newly rebuilt so the timing chain should still be tight.
4. Check the up/down play on your distributor. Should be no more than 1/32".
5. Don't lean out your carb too much, a lean condition will cause pinging. Try 1/2 turn more out and drive it.
6. Don't get too hung up on the actual timing degree numbers. Many racers time their cars by feel and what your particular engine likes. Called "power timing". Start at 9 degrees and move ahead a notch at a time and drive it. When pinging stops and you get the best throttle response back off a notch and you're done. No Timing light needed. Hope this helps.