Of all the people that should, or even could, answer this post, I'm not him....but.....

It occurs to me you definately want atleast 2 ground points. One to the body and one to the engine. The motor and tranny mounts have rubber isolators in them, so without the engine block being grounded your motor would never turn over....and remember the motor should have like a 4 gauge ground wire going to it alone. I imagine you would have the same problem with all the other electrical items since they are grounded to the body and not the frame....and the frame is isolated too.....although the body mount bolts might complete that curcuit.

Anyway.....just a rookie's prespective