From Gilbert Chevrolet:

Flywheels/Flexplates with two different diameters are used on Chevrolet small-block V8, big-block V8, and V6/90° engines. Large flywheels/flexplates are 14” in diameter, and have 168 teeth on the starter ring gear. Small diameter flywheels/flexplates are 12-3/4” in diameter and have 153 teeth on the starter ring gear. Flywheels are used on manual transmissions and flexplates are used on automatic transmissions.
The difference in flywheel/flexplate diameters requires two distinct starter housings. Starter noses used with large diameter flywheels/flexplates have two offset bolt holes; starters for small flywheels/flexplates have two bolt holes which are parallel to the back of the block. Most Chevrolet blocks are drilled for both types of starters. If you are changing the diameter of your Chevrolet’s flywheel/flexplate, you can convert your present starter to fit the new flywheel/flexplate by installing the appropriate starter nose, starter motor housing, found within each engine classification.

01968122 - Starter Nose. For 12-3/4” diameter flywheels.
Technical Note: Used with starter P/N 10496870. Bolt holes are straight across.

00354353 - Starter Brace 153 Tooth. Braces greatly reduce the load on the starter nose.

10465143 - Starter SBC and BBC, light weight, high-performance, Remanufactured. Used originally on Camaro Z28’s with LT1 engines, but can be used on any small-block or big-block Chevrolet engine with 12-3/4” flywheel. Price includes core charge.