The horsepower of a 1969 Chevy 327 small block with casting number 3956618 depends entirely on its specifications and modifications. The casting number alone doesn't tell you the horsepower. That number identifies the block itself, but many different combinations of heads, camshaft, carburetor, intake manifold, and other components were used with that block, resulting in a wide range of horsepower outputs.
A stock 327 from 1969 could range from approximately 210 horsepower to over 350 horsepower depending on the specific options selected. To know the horsepower of *that specific* engine, you'd need to examine the original build sheet (if it exists), or closely inspect the engine itself for clues about its components.
