Best place for ground under dash?

Originally, the front mount battery had the main ground to the engine block, but THAT is not well grounded to the body. Early cars had minimal battery or engine to body grounds. That was usually a pigtail off the battery clamp to the body

If you are using a trunk mount battery, are you running the neg cable to the body / frame rail in the trunk, or clear up to the engine?

If you are grounding the battery in the trunk, then you need a Great Big cable from the engine /trans to the body. If you run the battery neg up to the block, you need a not so big cable from the engine to the body

One good way do do this is to look at the alternator bracket bolts in the left front head. Those same holes are in the rear of the head on the driver side.

Depending on what you have for a master cylinder, you can buy a 1 foot starter cable (ring to ring) and bolt on the rear of the head, and to the master cyl. studs. If the neg. is already run to the block/ trans, you can use a no 4 cable. If the neg is grounded only in the trunk, use a no1 cable.

Just use your noggin a little

Used to be common to see the '60's Chevies with part of the tail lamps in the trunk lid doing crazy things---because the lamp sockets needed to be grounded.

"Back when" I had the 70 sixpack car, I forgot to hook up the engine to body ground (trunk mount battery.) and tried to start the car. "It" was trying to ground through the clutch linkage, and blew up one of the retainer clips on the linkage. I knew better, even back then, here was a clip that looked like it had been struck by lightning.

We were all ETs (RADAR techs) so I took a lot of flak on that one.