Funny every early Mopar Ive had since the 60s had the batt ground to the motor.The small ground on the back to firewall is to ground for the body not the starter.Sounds like either bad cables,bad starter,or motor locking up.The starter and ground are all grounded thru the block.I had a GM once new motor.Tried to start and smoke came up.Seems my buddy had the small ground wire attached but the large one that bolted to the alt bracket wasnt.Attached it and all was goog.Just because cables look good they can be corroded inside.Rocky