The high voltage low current flow that the plug uses should go though the engine (head), the coil (or distributor's coil), the wires (or just the coil on plug itself), and the plugs, that's it. The low voltage high current input from the battery that gets converted to high voltage low current would go from battery, fuse box, and a bunch of cables.
Unless your existing wire is corroded, adding additional engine grounding wouldn't do anything.
p.s. Ground can have different meaning depends on how you look at things. On a waste spark the ground could be just the engine block / head, and the non "ground" electrodes of the spark plugs that connects to the coil pack could be labeled "positive" and "negative", etc. Ground is just a name given to a reference point in a system of "flow", like electrical current.