Stripped bolt on firewall for Voltage Reg.

Had a bit of time so I worked on fixing the 65 Dart’s energetic amp meter. I replaced the old VR with a solid state which seems to have fixed the problem (electronic ignition orange box) but uncovered another problem. Someone stripped out one of the bolts connecting the VR to the firewall. I replaced it with a washer and screw for a temp fix but I’d like to fix it right. Considering that the VR needs to be well-grounded, Bondo or JB Weld isn’t going to cut it. What do people suggest for a more permanent fix? I’d like to keep it as stock as practical. Suggestions?
use a bigger Bolt