Success!!
This morning I filled the radiator, started the tractor and let it run through breakfast and morning chores. After 20 minutes there was no leak.
It has to have been that the heater just wasn't snugged up against the block wall tight enough as a result of the double wingnut being set wrong. It could have been that the O-ring was damaged in the first hasty 11:30 pm attempt, but I see no visible evidence on the old O-ring of any damage.
The first picture shows the side of the tractor after 20 minutes. You can see evidence of past leaks, but no fresh coolant. The second picture shows the cement under the tractor. Look Ma! No Leak!