71 Dodge Dart Swinger in England

Mopar had electronic ignition as optional equipment starting in 1972, standard equipment in 1973. ECU was same part for 6 and V8. It only records the pickup pulse to fire the plugs.

Yours is a 1971 , providing you really gut instinct feel this is an electrical related issue, i would strongly recommend going to www.mymopar.com and printing a copy of your wiring schematic, then dewire the added in electronic ignition wiring out of it. Then with a clean slate with all the stock wiring intact, and checked, then wire up the HEI conversion. I know thats what i would do.