Runs at FAST idle, not slow

Do you have a tach meter you can hook up to measure the rpm? At what rpm does it die? It sounds to me like the idle circuit in the carb isn't working, thus it only runs when the main jets flow. That matches your experience that the idle screws do nothing. A rebuilt carb isn't always fine. What model do you have? If the Holley 1920, I went thru 3 of those for 18 yrs, always idling rough, even with head jobs and finally a whole new long block and no change, until I got one that ran unbelievable smooth. I don't think anyone can rebuild the 1920 assured since it has hidden internals in the "metering block". I have a Carter BBS on my 64 Valiant slant and it looks more approachable, very similar to the venerable Carter BBD.