OK. By the way, I did see another article while searching the web that said spark plug intensity can be affected by how well the block is grounded.
The spark plug is grounded thru the block and the direct electrical path is thru the block to the ground cable from the battery to the block-not the firewall.
Items that are typically grounded to the factory sheet metal like the head lights, regulator, electronics box, etc., normally don't pull very much current and given the short length of wire involved, it does not require a large cable (resistance includes wire diameter, wire composition, wire length, and connection quality)
Given the age of the sheet metal, the firewall may not be as well connected electrically to the fender, etc., as it was originally and it always a good idea to check the voltage drop with a meter but seldom is a ground wire larger in diameter needed. If it is, it will show up on the meter as a higher voltage which means the current in the circuit is taking an easier path thru the meter because the direct connection is not enuf.
You have to take the time to understand the articles and put them into the context of the perceived problem in order to know if they may apply or not.
On new cars, messing with the factory ground circuitry can sometimes create ground loops which creates errors in the data being sent to the computer and performance may be affected. Of course, every Honda owner knows you can pick up two tenths by adding a $150 ground kit
On older cars, I have seen an error greater than 1 A/F caused in the sensor reading by not adhering strictly to the install instructions with regard to grounding
With the exception of ice cream, one can definitely get too much of a good thing
