Since the only difference in a 230 and a 250 is the stroke, the only 100% sure way to tell what you've got is to figure out a way to measure it without pulling the cylinder head. As you've discovered, the block castings are the same for both engines. A little creative probing down a spark plug hole with a thin, flexible tape measure might be a possibility. The 230 has a 3.250" stroke, while the 250 stroke is 3.531, so there's a little over a 1/4" difference. Somebody might be able to decipher the stamped number behind the distributor for you, if there is one there.
Jerry

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