It took me a while, but I eventually resolved the issue.
I did eventually increase timing, to either 12 or 14 initial (can't remember off the top of my head, but total is 36 at ~2800RPM), which seemed to improve the issue a bit.
I didn't mention it in the above posts, but once in a while, the car would start up just fine, idle fine for a while, then eventually stall out. When I tried to start it up again, it would just crank forever with no actual ignition.
At some point in the time, I decided to just say "screw it", and drive the car with what I thought was a high idle. I got about 60 miles out of it, and did notice that it was getting progressively harder to start up as the miles piled on between trips.
A week later, I tried starting it up, only to completely flood the carb (gasoline coming out the vent).
Eventually traced it down to a carb float bowl which had a microscopic leak in it, and was gradually letting fuel in when it was submerged. Once I replaced the float bowl, the issue went away.
Now the car runs fine with a 750RPM idle, and I have put approximately 1400 miles on it.
Moral of the story: Never assume that a rebuilt part bought from a reputable dealer (Jegs) is functioning perfectly. And sometimes, you just need to keep the project moving forward and troubleshoot on the road.
It's possible that the issue I described in my original post was primarily timing related, but I definitely think the leaking carb float was just adding to the confusion.
