Voltage rising then dropping? (67 Barracuda)

Scraping the paint off the firewall does nothing. Don't do it. It will rust there over time and cause you more trouble.

The automakers don't scrape the paint off the firewall of a new car, and they build millions of cars this way and warranty them for years. WHY and HOW does this work? Because the ground is through the fastener threads and head of the fastener to the part. So make sure it is snug in the sheetmetal and not stripped. Now use a star washer under the head of the fastener. Done, now you have a good ground.