I would go with the 600. Why not? most people don’t know how to shift a tight shift pattern after being used to a muncie. I do not know what kind of gears in the rear you are running. If they are 4 11, I would consider a .50 overdrive. The only real factor is cost and fuel savings. If you are going for fuel savings and want to spend extra money then I would go with the 56. If you want to have an overdrive for a long strait on a road course then the 600. My 2pennies