Grounding questions.

I like to use a BIG ground from the engine to the body in addition to "whatever was there." Notice the alternator mounting hole(s) on the front of the pass side head. This same hole(s) are on the rear of the driver side head. Use a short "starter" cable (eyelet to eyelet) maybe a foot long, and bolt to the firewall, or to the master cylinder mounting bolt

Grounding strategy gets a LOT more complicated the more electronics you have, especially EFI, O2 sensors, ANY modern systems using sensors, solid state, etc.

Part of the reason is noise pickup IE ......EMI --electromagnetic interference-- a great example of that is high output ignition such as MSD. There has been cases where the high power pulses in the plug wires is induced back into the pickup/ trigger circuit ad re-trigger the MSD.

Another is what is called 'ground loops' This is where current in a circuit or ground circuit causes a voltage differenc which then affects related or connected circuits. The infamous "up and down" pulsing of a voltage regulator is just one example. These can be difficult to run down.