I just bought a 1975 Powerboat with an OMC 235 engine. I was told it was the same as a Ford. How do I know if it is a 302 or a 351?

 

I have seen links to identify casting numbers, but haven't seen where these numbers are located on the block to identify it.

 

Can you post a picture?

The taller deck height of the 351 means the block is physically taller and wider than the 302 and this shows up in a couple easy to see places like the surface where the distributor mounts or the water inlet relative to the head gasket surface. The 351 intake is also about 11" wide while the 302 is only about 9" wide.