[68 P.O.S.]

I would use a piston stop and do it right to be 100% sure. My luck, I would go with my assumption and end up grenading the engine �� But if you're comfortable with it, cool.

Yeah, it sounds like you probably never had any vacuum advance going on. Good job putting it on manifold vacuum. No, 20 degrees vacuum advance is way too much. Yes, other people run 20 degrees, likely on ported vacuum, because they probably don't understand it. Distributors in the 70's/80's had high vaccum advance to make up for the late timing they ran for emissions. You want 8-12 degrees vacuum advance..

The next step now that you're on manifold vacuum is to get an HEI. Haha.

Tuning with a vac gauge is a good idea, but timing with a vac gauge isn't. I suppose you could adjust initial timing for best vacuum and then use a timing light to fine tune the total advance.

On a carbed engine, there is no definitive way to adjust timing and idle to perfection, besides adding fuel injection. Too many environmental factors that change everyday. You can get a wideband and a carb calibration kit and get damn close.