The screw on the throttle linkage is the throttle stop screw,it keeps the butterflies from getting stuck in the throttle bores,if this screw is cranked in too far,you rob fuel from the transition circuit.Your Idle speed should be mainly controlled with the screws at the base of the carburetor. Yes everyone uses the throttle stop screw to adjust speed but that's not its primary function and can cause drivability problems. Anyway you should check for vacuum leaks as many others have recommended but also familiarize your self with your carburetor it's different functions and adjustments. For all I know your Idling on the fast cam.
You've gotten some very good advice already. Someone touched on total timing,which you want to check. You should have approximately 36 degrees we can tweak it later. Total is base timing plus centrifugal,no vacuum advance hood up.You use timing tape or a dial back timing light to check it. Vacuum leaks,spray carburetor base and manifold perimeter,with carb cleaner you will hear the engine pick up if you find a leak. 2nd stage of vacuum leak checking, remove every vacuum hose from the carburetor and plug all the carburetor ports,hopefully you idle correctly then. Now start hooking things back up,1 at a time while listening to your engine. If it speeds up considerably when 1 line is hooked up,you've located a problem. I won't be surprised if you find something unhooked. And I do not agree with removing egr and it probably will make your truck run worse. The solenoid on truck quadrajet is just for a 2 stage accerator pump and won't affect idle.