There are two things that come to mind.

1st, and fairly common, the vacuum advance on the carb. I know when mine was shot it stopped fast idling when cold. Hook a vacuum gauge up to it and see if the diaphragms are blown (quite likely). Unfortunately a new one is about $100 bucks.

2nd, the fast idle screw may just need adjustment. It's below the warm idle screw, pointing downward at about a 35? angle. Tightening raises the cold idle, loosening lowers it.

Contrary to popular belief, while it is possible to adjust the chokes on these trucks, you really shouldn't. There is a specific way it's supposed to go on (a locking tab insures this from the factory), and your truck may protest if you tighten it.