[Monte Howard]

My first thought was the wires from the msd box to distributor, do not let them go anywhere near the coil, I know this has been suggested already. So I would look at the wiring/ switches you changed then I would throw a different carb on and see.
I have had two quadrajets over the years that went bad all of the sudden. They would not rev over 4500 rpm. Rebuilt them several times with the same results. Finally gave up and they are in the trash now. Don't assume the carb is ok, the only way to know for sure is by trying a different carb.
If a different carb doesn't work, I think I would rewire the entire car with a nice new switch panel.