Ignition problem

With factory electronic ignition the ignition start circuit has two jobs:
1. Feed the coil.
2. Feed the ECU.

IIRC '72 may have an additional engine bay connector for ignition start to run through. Its possible it got yanked or a cross wire short if pinched during the exhaust work. In the service manual ignition start (Ignition 2) wires are labled J3. From the ignition switch to the ballast resistor junction, they are usually brown.