You noticed (and nailed) the hardest part of driving a car with an open rear end. One of the trickiest parts of driving this car is the launch with the open rear end and a small tire. You are correct, it's a tight, stock, factory converter that came behind a 318. No preload, and the sudden flash-stall will break one tire loose, or in the very least hurt the 60 ft time. Too much preload, and it will break the tire loose. It is not a "set rpm" as I have no tach in the car yet, but strictly by feel on how much is "just right" so the car will launch without spinning getting the best 60 ft time the car can turn.
