Temperature gauge does not work on my 1970 340 Duster ?

The block to body ground is very important. The negative battery cables hooked to the frame and the block both carry current, so there can be ground loops and voltage drops that are not obvious. The ground from the block to the frame will not necessarily carry a lot of current, but will hold the block and the frame at the same potential. That is why it is usually a large gage wire or or mesh braid. As someone mentioned above, I used a battery cable with two ring lugs. I tied one to the block and the other to a firewall bolt on my power brake booster. Even if it doesn’t solve your problem it is good practice. Usually no fun chasing electrical gremlins.....