There is a pad on the block, right below that 1" letter casting behind the alternator that will have some stamped numbers on it.
If you see two rows of numbers, upper row starting with an "L", this signifies an original or marine issued block.
If there is one row of small stamped numbers, this signifies it was a passenger car block.
I would have lake tested it before purchase...engine problems become evident under load... Trailer running means very little since there is absolutely no load.
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