Interesting you should say that... earlier this afternoon I discovered what I think is the problem.

I pulled the carb and manifold and the only unusual thing I saw was a puddle of oil on top the valley pan.
No oil in the ports. Pulled the valley pan and it looked like some of the sealant had run down the pan, confirming a big gap.

I set the manifold directly on the heads and I immediately found that the "tips" were hanging up on the Chinese wall and I couldn't slide the manifold back and forth to line up the bolt holes without one end or the other hitting the wall. And when I had torqued it down previously, it deformed the valley pan there too like a sheet metal brake!

This is one of those "I shoulda thought of that"... with .055 milled from the heads, and my block has not been decked (it was really close when I built the short block a long time ago).

A couple minutes work with a die grinder and it then sat on the heads perfectly... So I put the valley pan back on with the bolt holes centered, and sealed the corners and the end rails. I also cleaned the Permatex #2 from the valley pan, as many people don't use sealant around the ports at all. Intake back on - only had to wiggle it a little to get all the bolts in and torqued to 25 ft-lb.

HOWEVER.
It still is running leaner and with lower vacuum, even after warmup. At least it will now idle without having to tickle the gas pedal, so that is an improvement. I waved an unlit propane torch around all the ports, especially underneath, but there was no effect on the idle at all. I didn't get a chance to see if the puff of oil smoke after decel is gone tonight.

Is it possible that my new, much better-breathing heads (made that much difference to the carb tune? I figure at idle, the throttle butterflies are the most important airflow limitation

Also the lash is tighter (set .006 tight for aluminum since the hot spec is .018), so maybe more overlap. That could definitely make vacuum drop, but from 8-9" Hg. down to 6-7"? I'll need to check one hot and see if it actually loosens up .006, as every engine is different.
Edited to add: I've been reading on other forums and it does indeed appear that it can make that much difference! I'll check one hot, but I think I'm going to run them again at .003 or .004 looser and see if my already-borderline idle manners come back

Any more thoughts? thanks.