Rusty is right. You need BOTH the body and engine grounded, but NORMALLY (front mount) the body doesn't have to carry starter current, so a minimal ground, an in my opinion, GM and Ma were both guilty of NOT ENOUGH of a ground from the battery to the body
(I use the unused head holes in the rear of the driver side head to run a starter cable from the head to one of the master cylinder bolts)
So far as the unibody carrying starter current, you have to remember that Mopars ARE unibodys, a singular, welded assembly, and assuming the connections at each end are good, you have a LOT of surface area there, even though just thin sheet metal. LOTS of people have successfully used the unibody as an engine ground path, with no problems.
Another issue with proper grounding is radio noise, and this same "noise" can also cause problems with other modern electronics. A recent thread on another board (where the OP just would not listen) involved an MSD which interacted with the charging system. He'd unhook the regulator, it was fine. Other sensitive gauges, like in over the road trucks ---EGT gauges, electric speedos, the list goes on, can all be affected by ground loops and other unwanted stray voltage "floating around." It's also the reason that breakerless distributor leads should be twisted, and those that remember TV antenna "twinlead" remember that from a LONG time ago.
And today, it's why CAT3, CAT5, etc cable pairs are twisted