"Problems: I can't get it to tune. The idle mixture screw has almost no effect. While driving, the car stall when I even just tap the brake, to the point that it is dangerous and undrivable. I've tuned it with a tach and vacuum. Idle mixture screw does almost nothing."
This does sound like a possible vacuum leak. Not too long ago, I worked on a 4.9L which idled quite high [~1000 RPMs], ran poorly, jerked on deceleration and @ idle speed, pulled 18.5" Hg. A comparable engine [mine] pulls ~21.0" @ idle. I tried every trick I knew to find the leak. I used carb cleaner and checked all vacuum lines on the engine, HVAC system, PCV valve and power brake booster. Using needle nose pliers, I pinched each and every vacuum line. I sprayed along where the intake manifold attaches to the engine. I even made a block-off plate for the EGR valve. None of this worked. One time, it seemed that when the carb cleaner was sprayed at the #5 intake runner [manifold], the idle would slightly change, but then it would not repeat???
I saw a trick on Youtube and went out and purchased a $0.69 cigar. I covered the mouth of the carburetor with duct tape [engine off, of course!] and took the vacuum tube which had been disconnected from the temp. sensor on the air cleaner. This hose was still connected to the vacuum tree. I lit that bad boy and took in a deep draw and blew into the vacuum hose. I placed my thumb over the open end, while taking another draw. With the second blow into the vacuum hose [that quick!], I could see smoke coming out from under the carburetor toward the engine. I couldn't believe my eyes. The vacuum tree is screwed into the intake manifold and has numerous ports, some unused. There was one which I had never seen at the bottom of the vacuum tree and pointed down towards the ground. [Keep in mind that on a 1984 4.9L feedback equipped engine, there are yards of vacuum hoses and wires obstructing the view.] Well, the rubber cap had a large hole in the center. This cap likely was the factory original and is nearest the exhaust manifold, so is exposed to more heat. Since it was oriented downward, I could not see it, hear it or hit it with the carb. cleaner spray. In hindsight, the time I thought the idle changed, the spray must have bounced off the engine block and partially hit the open port.
Prior to replacing the rubber cap, the idle air mixture screw was turned out ~5 turns. Immediately after the repair, it needed to be turned in ~2 1/2 turns. The vacuum increased to 21" Hg and the deceleration jerking stopped.
For 1/2 of a cigar [~35 cents], it was a cheap and quick fix.
I hope this might help and I apologize for the very long post.