I have a 350 HO engine w/700r4 tranny. Pre 79 emissions engine, the problem I'm having is idle speed. When ac turns on and I pull it in gear it stalls, if I idle it up to stop that it won't turn off. Someone told me I needed a fast idle switch install??? Any help

They are correct. Solenoids are available for most carbs that kick the idle up a bit when the A/C is on. Solves the problem you are having at idle. If you have a GM carb search the junk yards or post in the wanted section. If you have an aftermarket carb go to the manufacturers web site and search around.

https://www.napaonline.com/Catalog/CatalogItemDetail.aspx?A=CRB22081_0205551574&An=599001+101985+50014+2014016
I believe this is what the parts stores call it,a throttle stop ,like what was said before when you turn on the A/C it raises and stops the throttle from closing too far and maintains idle.