Question?

The "main ground" (big current) is direct from the battery to the engine block. The starter grounds to the block. The alternator output is grounded to the block.

From there you need a good ground from engine to body. What I like is (v8) on your driver side head, are unused holes in the rear of the head. Buy a 1 foot or so starter cable (eye to eye) and bolt to the head, and to one of the master cylinder studs, or to a good through bolt through the firewall. This insures grounding for lights, the voltage regulator, and the electronic ignition if used.

The K member may not be a good choice. It is held to the frame with 4 bolts which might be rusty Remember, these cars are unique because most of the body/ structure is a one piece welded assembly.