Don’t be concerned with under hood heat possibly being the cause. Crap coil (highly likely), mismatched ballast resistor, suspect grounds and/or connections more realistic. You did check to make sure the BR was the correct one for the coil that went bad? And the one you have now is the correct one for the new coil, right? Something to keep in mind (if you hadn’t given thought).....source a ballast resistor that matches what ever coil you use, don’t just buy a ballast resistor for a 72 Dart.