Sean!
Is it actually boiling, steam, horn sound from radiator/reservoir cap, etc?
Or are you just going by the temp gauge indication?
I think you have the timing set right for optimum temp and minor changes
should not change the temp much. And it doesn't make any difference where
the gear is positioned as long as the spark is getting to the right cylinders
at the right time. I don't think the mixture is the problem either if it starts
and runs OK. If the thermostat IS opening and the hose is NOT unduly
hot, I submit that the engine is not unduly hot either. You could put a
thermometer into the radiator upper tank, and if you are getting water
circulation, the temp will not be much different there.

The ONE other thing I can think of is that perhaps the brass water distribution
tube inside the head is missing or damaged. But that is a pretty long shot.
It is about 7/8" in diameter and runs from front to back, in the water jacket,
and both ends are covered with a freeze plug probably about 1½" in diameter
on each end of the head. You can see it through the thermostat/outlet flange
with the outlet and thermo removed but you may need a light and a small mirror
down inside. The tube has slots that spray a stream of water against the
exterior of the combustion chamber so the coolest water goes to the hottest
place.
--
George Downs Bartlesville, Heart of the USA!