No power while cranking

1974 dart 318. Ok, while cranking I have no power to anything until I let go of the ignition out of the cranking position, then it starts sometimes. It usually backfires through the carb as soon as you let go of the ignition. If it does start, runs great but keeps shutting off. Most of the time it don't die completely, just enough for the heater and wipers to lose power and stumbles back to life. Driving it does the same thing, all gauges go to 0, heater stops then instantly goes back to normal. I changed the ballast resistor, ignition module, starter, starter relay, coil, wires, cap and rotor. The car sat in a garage covered up for 32 years. I cleaned the 3 plug ins at the firewall and changed both battery cables. I just ordered an ignition switch and it will be in today. Could the switch be the culprit?