Yeah I have thought about all of the ways that coolant could have ended up in the engine, so I'm hopeful that it would be something more simple like that.  It's the fact that the motor just up and quit when this farmer was using it is what makes me wonder.  He said that they had the head checked out and that the machine shop said it had a crack in it.  So I'm basically just going by what he is saying for now, until I can look at the tractor. 

I had seen on a few other forums that a 550, and super 66 head will fit this motor as well?  Is this true?  I've never done any work on an Oliver before.  My family used to have an 1800 when I was a kid, but we sold it when we got out of farming.  Loved that tractor.