Manifold Vacuum Advance what am I not getting?

Unless it is a stock engine, MVA shoulds ALWAYS use an adjustable VA unit. There are adj units [ Crane, Accel, Mr. Gasket ] that have a spacer inside the spring. Have only seen these on GM units, it is a most stupid idea.......because you can lock the thing solid when adjusting the Allen Key. With these, I prise open the can & remove the spacer.....
Here in Oz, it is almost as cheap to buy a Chinese dist [ comes with adj VA unit ] than it is to buy the VA unit on it's own.