ignition resistor question

You didn't give us much to go on. Assuming the 78 has the factory Mopar ECU, you do need a ballast resistor unless your "NAPA coil" is a "ballasted coil", i.e. primary resistance >1.5 ohm. This also assumes your ECU is either a 4-pin type or the 5th pin is a "dummy" (infinite resistance to case) as most new ones are.

If you require a ballast and don't use one, it will run fine (even better) for a short time, but the coil will get too hot after ~5 min and start missing, especially under more throttle. No ballast may also overheat the ECU, but I am not positive. Can't touch the coil and smell paint melting = "too hot".