Tom,
Turning the idle speed screw in even a half turn will really effect the idle. A full turn will pretty much stall it out. Little effect on the vacuum however.
I still need to plug the line to the doors & check on the tranny vac line.
I pulled the ECU and looked it over & couldn't find the idle speed screw. (If memory serves me the ECU is the front on the left fender...not sure I could miss it
). Possibley on the '75 it is somewhere else?
A few more vac checks and we go for a drive...