75 318 Dart won't start and backfires!!!

I had pretty much this exact problem and it was my ballast resistor. The backfires were so loud when I turned the key off! I think with the low voltage the coil doesen't have enough juice to produce a good enough spark in sucession but, It stores up enough when your not cranking to make one strong spark. Then when you turn your key off the coil discharges and sparks and lights the massive amount of unburned gas in your exhaust and you get a big boom.