Why does my 66 mustang die at idle but runs great on the throttle?

carb may be sucking air through leaks , there may be other vacuum leaks , may need a rebuild

That sounds like you have crud in the carb, most likely needs overhauled. Back then we would put idle up solenoids on the carb and set them at 650 RPM in gear with the solenoid energized in drive and drop the de- energized rpm to 450 in neutral, they helped with after run problems, commonly know as dieseling.

Brand new carb, new distributer, new plugs and wires. I think its the timing. The dum dum that built the motor put the harmonic balancer upside down.

I put electric ignition to replace the old points. Thanks for all the input!

You can’t put a harmonic balancer on upside down as it has a key way, I have seen balancers go bad and the timing ring slip or that fall apart but that’s it. Double check the carb settings, if it is new it can be easily adjusted but if it’s rebuilt then you probably got a piece of junk, same for the distributor, they never check bushings or breaker plates during rebuilding. So you maybe right about timing as it can’t be set properly due to distributor problems.