grounding question re: the neg battery cable
... rubber motor mounts?...
Pretty much yup. Typical older factory setups had the main battery cable hooked to the block, and a jumper from the battery to the body, and sometimes a jumper from the block to the frame/ body. Both these jumpers were typically no10 or so wire
VERY important that the body/ frame and block be well tied together, especially nowadays with all this electronics. Electrical "noise" can cause all sorts of problems in all sorts of equipment, ignitions, computers, etc, as well as the age-old problem of the regulator putting out too much voltage because it does not properly "see" the battery