I recently built a Muncie/BW R10 overdrive for a guy in New Zealand. I installed the HD close ratio gear set (Corvette/Pontiac) which has better ratios than the standard set up. He is using it in a 65 C-20 with an overhead camper. He upgraded the engine to a 350 and retained the corporate 4.57:1 rear end. It works well with that combination.
Fred