I would check out the ignition switch , the ballast resistor or resistor
wire / fusable link (depending on what was used at that time), any
capacitors attached to the coil for noise suppression, and grounds in that
You are telling us that you are bypassing the circuit that provides voltage
to the coil to create a spark becuase the voltage is too low, correct ?
You might also want to check all the wires ganged up on the starter
solenoid. Corrosion buildup between the ends could cause trouble.