Rough idle but drives very smooth.

I have never had a pickup coil come and go like that...(idle bad vs driving good) When they go...they typically die entirely, then magically work an hour later (in my experience)

how do you know you have a misfire at idle? just a weak timing light signal? have you physically looked at the spark with a plug grounded on the engine?

wouldn't shock me if the mechanical advance in the 60 year old distributor was getting hung up maybe? or the shaft was sloppy...or the block bushing.



Bad ECU or grounding?

I'm not saying throwing parts at it either, but you're under $100 to attack the distributor and ECU combined, or less if you do a module instead of the Dist...but make sure the distributor shaft isn't a wobbly mess. or even the cam bushing / gear.

Complete distributors looks to go for around $70
1976 DODGE DART 3.7L 225cid L6 Distributor | RockAuto

or if you want to try a module first, those are $15
1976 DODGE DART 3.7L 225cid L6 Distributor Pickup Coil | RockAuto

ECU $18-40
1976 DODGE DART 3.7L 225cid L6 Ignition Control Module (ICM) | RockAuto