[Terry, NJ]

Here's my solution! When you check the ground path, you'll note that it runs through some the original frame rivets and that could be problematical. The thing to do is to use a brass5/16 or 3/8 bolt through top of the chassis flange. Attach the ground cable from the battery on the top of the flange and run the new cable from the underside of the brass bolt over to the bottom bolt of the starter. The reason it ran so good for you on the bench is that the current didn't have all that rust to make it through.
I had been hearing about Auxillary Ground cables for years so I decided try one out. I attached a 12V (yes! a 12V cable) as described above. I couldn't believe the difference in the speed of the starter. Night and Day! Coincidentally, this also means that batteries will charge faster, the charge will give more spins at higher RPM, and will last longer. The draw down is not as severe.
Terry