If your starter spins but it won't turn the crankshaft, all it needs is a new drive gear assembly. If you're determined to replace it, count the teeth on the flywheel. A 139 tooth flywheel needs a 6V starter. A 168 tooth will need a 12V starter. It's OK to run a 6V starter on 12V as long as the engine is well tuned and it doesn't require a lot of cranking. If the engine won't start in 5 seconds or less, tune the engine so it starts easily.
The starter in your photos appears to be a 6V unit, which would be incorrect for a 1956 engine. 12V Delco-Remy equipment had a red ID tag. 6V equipment tags are black. Mid-year 1955, Chevy switched to a 12V electrical system, so your engine would have been factory equipped with a 168 tooth flywheel and a 12V starter. Count the flywheel teeth, as a 6V flywheel and starter might have been installed on a later model engine at sometime in the past.
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