The ballast resistor is behind the speedometer, it's a special wire with plug ends on each end , but I would check the plug at the ign module before you start removing things , you have a two wire plug with a red and white wire , one is hot when you turn on the ign switch , the other is a bypass from the start solenoid that bypasses the resistor to keep the voltage up when cranking , look for corrosion in the plug , (green ) if no volts with key on , bad switch or as you say bad resistor , (rare) also could be a bad box , if it is org they had problems back in 75 to late 76 , look for a blue tab one the box where the wire come out , that was the fix back hem , later Blaine