I do have a 4-speed, but still interested in a stable idle ;)
Your vacuum advance analysis makes sense... but remember my engine only idles at 8" (at best).
Cruise is about 14-15" @ 3000 rpm. A light push on the throttle drops the vac way down... and of course under heavier load there's little if any vacuum, so no advance either. The can could be limited to only add a maximum of 10 degrees or so, too.
A couple of Mopartsians have said they're running one light spring (only) so all-in once the engine starts. Both say they didn't have a problem on the street. I'm inclined to try a compromise as noted (and quoted) above.
Why would the idle be a little unstable with that setup?