David, I'm not certain of the casting number but I have a flywheel made for an 11 inch pressure plate (and correspondingly as you would expect the 11 inch clutch). It is the 6 bolt style. These were used in heavy duty applications and the one I have was removed from a totaled 1954 Corvette. If you can find one, the larger size is nice if your pickup is being used as a truck. If your pickup is not being used to haul loads of any consequence, as Jerry said it won't matter. Interestingly the Astro Van my five speed transmission came out of used an 11 inch clutch if it was the heavy duty van with the V6, so the clutch plate that matched the transmission input shaft wasn't too hard to find. Good luck.
