Topher:

In your post of 9/17/12 you asked about the influence of valve condition and adjustment. Have you adjusted the valve clearance and checked the compression? If compression is low or uneven, it may be quite difficult to resolve the timing and idle issues you describe.

The distributor which you currently have provides approx. 20 degrees of mechanical advance, and 20 degrees of vacuum advance. The total advance which you observe, 40-43 degrees with the vacuum advance connected directly to manifold, is appropriate for the application. That total advance is observed in a low load condition, hence manifold pressure is low (high vacuum) and that much advance will not lead to detonation. When the throttle opening is large, and the engine is under load, manifold pressure will be high (low vacuum), and the vacuum advance mechanism will provide a measure of retard to bring the total advance into the low to mid 30s, which is safe. I would hold off using a "mechanical only advance" distributor, for to be safe under load, the total advance will have to be set in the low to mid 30s, and you will not enjoy the mileage benefit of increased light throttle advance.