Do you have a resistor or resistor wire in your ignition circuit (between the switch and coil +?
My 79, with the small cap electronic distributor and matching igniter/coil were fed by 7v, dropped down from 12v via the resistor or resistor wire.
The big cap FJ60 distributor, igniter and coil are fed by full battery voltage (no resistor or resistor wire in the ignition circuit).
When I had problems with my 79 OEM distributor, I opted to go full upgrade to the FJ60 setup (distributor, igniter, coil, dented pushrod cover), and I eliminated the ignition resistor wire to give the setup the full 12v it needs.
In your case, you can probably just pop in the big cap distributor and dented pushrodcover (verify your harness and distributor wiring matches up correctly), keeping the rest of your setup in tact. But you might consider the full FJ60 ignition upgrade for long term performance and reliability.