Didn't someone identify that "M" in the circle as the identification mark of a foundry in Canada recently? How about the oil ports on the side of the block opposite the starter? If it's got two 1/2" pipe thread plugs there for full-flow filter plumbing it's definitely a 261, but I'm not sure if that began as early as 1958.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!