ECU box ground.

I see nothing wrong with an added ground. While you are at it check your charging/ running voltage. Two issues "usually" plague these girls. 1, mostly harness voltage drop to the VR causes overcharge. 2, sometimes the VR ground. "A good plan" is to look at the pass side front of the head. Those holes are on the REAR of the driver side head. Use a short "eyelet to eyelet" starter cable, 12"--18" long, bolt to the head, and a good point on the firewall, the bolts mounting the master cylinder on manual brake cars is good.