News news news! Apparently I've been mistaken. I had my vacuum advance hooked up the way it was from stock with the 305.
Initial timing with vac advance disconnected, at idle, out of gear is 12*
Connected, it's 30*, at idle, out of gear.
It must be a manifold source at the carb, since it's advancing 18* at idle with vacuum advance connected.
That said, I ran the tests you asked me to, to figure out total timing.
Vacuum advance disconnected, I revved it to 3500, at which point it stopped advancing, staying at 30*.
Vacuum advance connected now, I revved it to 3500 again, and it stopped advancing at 50*.
I checked the manifold vacuum at idle again, and noted that indeed, it was around 14-15* at an idle roughly 750-800. Again, it idles very rough.