My understanding is the flywheel must match the bellhousing. This is because the starter must be in the correct location to mesh with the gear teeth on the flywheel. Once you have determined which size flywheel is needed, then you can determine what size clutch will bolt onto the flywheel. Something else that must be determined is what size disk is available that will fit the input shaft of the transmission. Then you must verify the clutch disk is the correct thickness to work with the pressure plate you intend to use.

If either an 11" or a 12" clutch will fit the flywheel and the input shaft, then you have the option of choosing which size clutch to install. The intended usage of the truck should be taken into consideration. Back in the old days, the general rule was that trucks used for heavy duty purposes had a large clutch. Performance cars usually had smaller diameter clutches and flywheels. (Allows quicker shifting and the engine to rev faster.) This advice may be outdated, so it may be wise to get some current advice on this matter.

Blown340, please tell us how to install a Dodge input shaft into a Chevy transmission.