As long as it has enough wattage capacity to handle the current load, you should be OK. Watts = Volts X Amps. A resistor that drops 3 volts at a 5 amp current flow will need to have a minimum of a 15 Watt rating. A 25, 50 or 100 watt 1.5 Ohm resistor would give you plenty of fudge factor to prevent overheating.
With the engine running, you should be able to measure about 3-4 volts by putting a voltmeter to both sides of the ballast resistor. The voltage applied to the coil (+) terminal should be around 9 volts if everything is working right, assuming a 12 volt input from the ignition switch. At full charging system voltage of 14, the voltage drop across the resistor will be a little higher, maybe 4.5-5 volts.
Jerry
