After much hemming and hawing and tinkering, I've reached a point of success. It took a while, but it was worth it, and I'm grateful for the advice I got from everyone on and off brick board. I couldn't have done it without you.

I got the timing dialed in. She likes 10 BTDC, 30 max advance, with no pinging whatsoever. The timing is holding steady now too.

She still wanted to idle above 1000 rpm, and she burned clean two flats richer on the highway than in town. At first this drove me nuts, and I kept wondering what I was doing wrong. I mean, I checked everything a dozen times, but no vacuum leaks. And I could get her to give me a really nice looking tan plug, but only at one fuel/air setting on the highway, and another in town. Talk about frustrating.

And then I talked to the guy I should have talked to all along. He's a long-time vintage Volvo whizz, and he explained something I wish someone had told me months ago. With a B21 bore, full-on street-performance-ported head, VV71 cam, and, wait, what was that? SU's? Everyone say's SU's will work on a B20. Yes, they work great. but not with static compression at 195psi. On the one hand, the vacuum this motor generates simply overwhelms the efficacy of the SU vacuum pistons, and on the other hand, the dynamics in the ported head generate fuel distribution effects that are somewhat incompatible with today's auto fuel. I'm told vintage Volvo motors ran 100 octane fuel, and liked it.

The solutions:
1) Throw in some octane boost.
2) Or install fuel injection (no, not D-Jet, more like something you can tune with your laptop.)

Well well. What do you know.

Anyway, I'm on down the road one happy camper, grateful for success. Thanks again, everyone, for your tidbits of sometimes-contradicting advice. What a journey!