I believe the block might be a 302 or something else (see here: https://www.vintage-mustang.com/threads/1968-289-spark-plug.613074/)
The plugs are autolite 25's, not the 45's I would expect for an older 289.
Reviving this old thread. I'm the latest owner of the boat. The plugs looked fouled, I'm trying to figure out what the appropriate plugs are for the engine. Is this a 289 windsor or a 302? It's hard find a casting number that makes it definitive.
unknown interceptor engine (289?)