I went with the O-ring because I could not find a fixed piece with the right configuration. The radiator fan control is a dictating factor.
I did apply a thin film of Permatex to the block surface and to the O-ring, hopefully that will increase the likelihood of integrity.
Of course, I get to undo all that to correct my dumb mistake of leaving out the thermostat. (I planned to do that today , but the surf was too good to pass up.)
