People commonly define Chevrolet Bellhousings by the Clutch size i.e. (10.5",11", or 12"). Flywheel Diameter (12.75" or 14") or number of teeth is more accurate because some 168 tooth flywheels are drilled with multiple bolt patterns and the aftermarket makes a 153 tooth flywheel that a 11" clutch and pressure plate will fit.
Chevrolet has always had just two sizes of flywheel Big (168 tooth 14" diameter) and Small (153 tooth 12.75" diameter) with the 153 tooth flywheel being introduced in 1963.
The early bells were cast iron with the aluminum bells being fazed in and eventually the only material for all factory bells.
With that said most Chevy Bells will physically fit Small blocks from
1955 on up, 6 cylinders from 1963 on, and all Big Block engines, W
engines, as long as they are Chevrolet engines. GM used engines from
other divisions in Chevy cars. But there are major differences
to consider such as transmission mounting hole size, clutch fork ball
stud locations, z-bar mounting provisions, starter, clutch fork hole
location and of course Factory correct application.
168 tooth 153 tooth
Descriptions of part numbers.