Richard,
The Era Correct Part Number for your Vacuum Modulator is GM #3027842. It was later superceded to #8629939 after 1982, which is the smaller type modulator which GM offered as a replacment. I've been dealing in GM parts for many years. I have a full library of GM parts master books for all GM Divisions from 1940's -1990's. Let me know if you need any other part numbers.
Aaron
How about a climate control....