Fluctuating neg. to ground on ignition coil

Being in Sweden, I don't know how quick and easy it will be to locate a dual ballast, which is needed since its the original ECU.

The resistor for the coil can be bypassed for a short period of time. Normally the biggest risk is overheating the coil.
But with the dual ballast and original ECU it wouldn't hurt to look at how its wired. Running should not be an issue but starting might be.
I'm not really sure how those 5 pin ECUs get power during start and if the lack of resistance could hurt it.

I think safer is to get a normal 0.5 Ballast resistor if you can't find a dual with the proper resistances.
Then put jumpers to the 0.5 ohm resistor and the 5.0 Ohm resistor.

There are ECU testers, but they are rare to here, Iprobably more so in Sweden. Some are just for Chrysler, but others have adapters for all major ECUs from the 70s and 80s. Anyway. One thing at a time. The resistor is probably out of spec, investigate that first. If you got a '79 Dodge truck shop manual, or even a generic shop manual for the truck, the coil, ballast and other specs should be in there. Sorry I have nothing that new for mopar.