The main difference between small block 221 260 289 302 351W and 351C bellhousings is the with the flywheel size. 157 and 164 tooth are the 2 versions of flywheels and each flywheel requires a specific bellhousing. If the wrong flywheel is installed with the wrong bellhousing or visa/versa either your starter wont fit because it will simple hit the flywheel on install (do not force the starter in the hole) or the starter will just spin into free air when 157 tooth flywheel is installed in combination with 164 tooth bellhousing.

One of the ways each bellhousing size can be identified is by looking at the starter position. The starter on the 164 tooth bellhousing is mounted about .500 inch further outwards as compared to the 157 tooth bellhousing

Here both types of back plates are measured and compared to illustrate the differences

Another way to quickly figure out which is which is to measure between the last bolt on the passenger side bellhousing to block mount and the top starter bolt. This identification method seems somewhat easier as a 5/16 of an inch in difference between the 2 patterns is somewhat easier discerning the difference over a total of only a few inches.