This is the cans advance, it can be anything from 8 to 24 degrees. A 10 degree vacuum can is ideal. It is available from AC Delco Parts, or your dealership. Manifold vs Ported, its a never ending debate. Depending on the age of who you ask, and their own experiences you still have a 50-50 chance of one or the other.
Ported vacuum is the choice of the factory.
It is also my recomendation to start with ported vacuum. Depending on what level of modifcations you have done to your engine, your driving style, and your vehicles characteristics its a point of debate. Each presents its own set of advantages and disadvantages. My suggestion is to just try both and observe how your engine behaves for your driving style. Obviously, this only applies to those of us with the 10SI unit.