I have done a hundred or so vacuum conversions, and one thing I will never do again is install a used vacuum booster or master cylinder.
For one reason at least 1/2 of the used units I have installed were not good to begin with or fail shortly thereafter. Consider they have been around used sitting or in use for 30 years.
Second, you can go to a parts store and get a complete reman booster and master cylinder for a 1987 Regal non-turbo unit with a lifetime warranty and a perfect fit.