66 Dart electronic ignition issues

Ground needs to be minimum of no10, and bigger is better.

Here's the deal on a "stocker." The massive current is of course the starter, so the main large cable should go directly from the battery to the engine block. On V8's (not familiar with slant bolt holes) I like to run a no 4 starter cable (ring to ring) from the rear area of the block/ head to one of the studs on the master cylinder.

In your case, I'd take some no 10, or maybe parallel a couple of no 10's and run from a handy bolt on the engine over to one of the bolts on your ECU, then run a short "jumper" no 10 from there to one of the bolts on the regulator. Scrape the front/ rear of both boxes bolt hole areas clean, scrape the fender clean around the bolt holes, and use star lock washers to re--mount. I guess those are "through" bolts and nuts? I'd use two per bolt--one under the fender and one on top of the ECU and regulator on each bolt, so total of 8 star washers

When you get it running, I want to direct you to a couple more tests, and you'll begin to see how important this stuff is.