Fixing stripped ECU screw for proper ground???!!!!

Since you can't get your hand behind it to use a nut, and you really don't want to drill it out larger; you have 2 options:
1.) Use a metric (oh, the horror!) screw that will use a slightly larger thread size- only you will know, and it's closer than going to the next SAE size.
2.) Take a small piece of copper wire, bend it into a "U" shape, and hook it in the hole. Then reinsert your original screw. The extra material of the copper wire will make up for the slop in the hole and allow the screw to tighten up.