Hi - I'd like to ask a quick question of BYOH20. You mention 2 ground wires off the "-" terminal. I'm having a Vintage Air A/C system problem and grounding is huge for those. I only have a cable running to the block but no second wire to the fender. Can you tell me where exactly it connects to the fender (I assume you mean the passenger side fender). Thanks in advance, Bob.
While you can wait for an answer from BYOH20, on the reproduction battery cables (pictured below) for my 68 Chevy II Nova I have two wires off of the battery post terminal. The heavier gauge wire bolts to the motor to supply the ground for the starter, alternator, and whatever else needs a ground on the motor or transmission. The smaller gauge wire on mine connects to a bolt on the backside of the radiator support qhuch then ties the body of the car to the battery ground and engine ground. I have had other year Nova's that this second ground wire bolted to the inner lipped section of the fender by the battery.
The reason for this second or smaller wire from the battery negative to the fender or radiator support is to better electrically tie the body to the battery and alternator.
You have to think how things are or were built on cars in that most engines and transmissions are supported by rubber mounts and rubber being an insulator does not pass electricity easily. The motor and transmission and the drivetrain does have an electrical path to the body but it is a poor one. Since the motor and transmission are on rubber motor mounts there is no connection there electrically to the frame or the body of the car. If we continue further on with the drivetrain, out of the transmission is a driveshaft and then to the rear axle and most rear axles being mounted to rubber mounts clamped to the leaf springs again there is no electrical connection to the body of the car. Now Past this we do get an electrical connection but it is poor and this is from the emergency brake cables. If the car has a mechanical or cable shifter tied to the column or to the transmission tunnel by the shifters mount, this too can be an electrical path BUT these are POOR electrical paths. Engine bearings, transmission bearings, rear axle gears, and sheathed shifter cables are not meant to carry electricity BUT they do but not as well as a wire or cable.
So with the above said, ideally run a dedicated wire or cable from the battery negative post to the body of the car.
While you can make this ground connection between the battery negative post to the fender or radiator support, you may find out there are issues doing so as I have seen fenders taken off cars to get repainted or replaced and then when bolted back to the car, the fender is mechanically tight but the paint prevents an electrical connection between the fender and the firewall. I've also seen the same with grounding to the radiator support which then bolts to the fender and then bolts to the firewall. While it's critical to have these parts mechanically bolted solidly together, they also need to be electrically ties together and sometimes adding grounds between the radiator support to the fender and then from the fender to the cowl, and even additional grounds between the engine and firewall can only reduce ground issues.
Also too when making ground connections, connect it to clean and bare metal and add a star washer between the terminal end and the metal it will bolt up against so the teeth of the washer digs into the terminal and the metal for a better connection than a connection made without the star washer. I also like to try and nut and bolt the connection instead of using metal thread screws as sometime you cannot get them as tight as you could with a nit and bolt type setup.