Constant spark from coil

Pretty much the ecu I think. It might be that some odd wiring problem is causing it to start a feedback oscillation loop. You might try regrounding the box--scrape around the bolt holes underside of the box, and reinstall with star lock washers. Make certain there is a very good ground from either block to body or battery to body. "Work" the distributor and ECU connectors in/ out several times to scrub the terminals clean.

Doubtful the ballast could cause this.