Yup. The aftermarket HEI distributors have adjustable vac advance cans so you can set it to be what you want, since there were so many variations in stock cans and they don't want to have to stock 97 different products. The problem with anything adjustable is that it must be adjusted. I would just go with a stock can, then all this rigamarole about how to adjust it goes away.
The vac advance is because, when there is vacuum in the manifold, this results in the gas molecules being farther apart in the chamber, so the charge burns slower. It therefore needs to have more time to burn, so the spark need to fire sooner. Since timing is in degrees, not milliseconds, as the rpms go up, a certain amount of degrees is a shorter time, so the spark needs to fire sooner in degrees to have the same time in milliseconds for the charge to burn. Thus the need for mechanical advance as the engine speeds up. We talk about ideal points -- base only, base + VA, base+ mech -- but in practice driving around, you almost always have base+partof the VA+part of the mech.
One note: If the VA is partially in all the time, even at 0 vacuum, that gets adjusted out when you set the base timing, so it isn't an issue. The issue occurs when it comes in too soon, or too hard.
Buy the stock can and throw it in there. Done. One less thing to worry about.
