in my experience all mopar products need a good ground, I had a stock 318 once in a truck that I painted the engine in and when I went to start it the cables got so hot they would smoke, cleaning the paint off under the ground helped and I have remembered that since.
it could very well be your starter I would start with a starter current draw test where that won't cost much if you don't have the equipment to do it, then if thats okay run a full ground to the front or at least try to make it better somehow.
