Since you started playing with the base idle screw, you need a scanner to see what it's base idle is. I use a Snap-On MT2500. Luckily though there are several write ups or pages with the correct information as to what your base idle should be and how to check it. Essentially you need to safely force the IAC motor to close by grounding out some terminals on the ALDL connector. Once it's closed, you unplug the IAC motor and unground those terminals. Connect your scanner, try and start the truck and see what the base idle is. Adjust as recommended. I believe 600rpm is a safe number to try with a 305/350. Then you can flip the key off, reconnect the IAC, start the truck up and see what it does. There is a special procedure to get the computer to relearn the IAC's position, sometimes that is necessary, hasn't been for me. But it sure sounds like you've tried to back off the base idle so much the throttle blades got stuck in the bores. Could be the issue, at least part of it.
Keep in mind the idle itself is electronically controlled & governed by the ECM. So unless you tune it yourself as others have mentioned with a custom chip, it will always try and target that number no matter what you do with the base idle. If anything, you'll only piss the ECM off by adjusting the base idle out of spec, as when you drop that, you're also adjusting the base voltage the ECM sees for the throttle position sensor. Annd if the base idle is too low, when you fire the engine up, especially in cold weather, it'll hunt/surge until it can find a happy spot, if at all. My 454 TBI truck did this because someone else must've thought the idle was too high, which it wasn't per GM engineering.