If the ignition switch power checks out good try powering the ignition circuit only with a separate battery. This means leave the ignition switch off, and add power to the ballast resistor with a jumper to the other battery, and negative connection to block. Use the original battery to crank the starter by jump power to the start terminal of the neutral safety switch.
This will separate the cranking and ignition power circuits for troubleshooting.
1. If the engine fires up, the ignition circuit is fine.
2. If no start, make sure there is a ground from the block to the control module, and try again.
3. Is no start, disconnect field wiring to alternator just in case the brush is shorting.
If the engine won't start, the issue is somewhere in the ignition circuit.
If the car starts at #1 above, your battery cables/connections have high resistance. Replace battery ground cable.
If car starts at #2 above, you likely needed the engine to body (ECU) ground.
If #3. have the alternator checked, and inspect the brush holders.
The why do this? The ECU needs to detect the signal from the distributor pickup. In detecting the signal, the ECU needs a ground reference for the signal.
If the battery ground wire has too much resistance, the starter current (Amps * resistance = voltage) will raise the "ground" voltage at the engine block, but the ECU is grounded to the firewall which might not be the same "ground" reference as the block. When releasing the key, the high starter current stops, and the ground references are much closer because there is much lower battery ground voltage drop (lower current = less voltage drop.)
This is for when you have already checked all the obvious things, and get really stumped.
