Timing running at 20* advance...

I think you've got a timing-indication issue here. The fact that you could make it run great without ping at 20° initial suggests the actual timing is different to the indicated timing.

Since you had to move the new dist a few teeth off your indexed location to get it to start, I wonder if you might see more realistic indications with the timing light hooked to a different plug wire (which in turn makes me wonder if your plug wire positions were off by one on the new cap vs. the old cap).

A simple indication issue like this is much more likely than violating basic principles about how engines work and still having it run great and not ping. You ought to chase this down and squash the bug so you can accurately dial in your timing, or at least know what you've actually got.

Speaking of distributor caps, read this entire thread. It's long and gets a little dull, but there's a big reward towards the end.