273 Can't get too much timing - confused

If you set it at 20* at idle, that doesn't mean an awful lot. At 650 rpm or 1100 rpm could be a big difference.
Check the timing at rpm. At the slowest rpm, then when it starts advancing. Then get at least 3 or 4 more rpm and timing measurements up to 3000 or 4000 rpm.
Do this with the hose ( for vac advance) plugged. Then we'll have some idea what's going on.
Its not going to ping under light load. You may hear it ping at part throttle moderate load esp with vac advance and too fast of a curve. But you could burn a piston at full load or close to it if its bad enough. That's when you've got to get out of the gas immediately if you hear it ping.