61 Sub
This is just a thought after reading what you've done to try to cool this motor down. Maybe the motor is making to much heat.
I had a 66 Tempest 400ci once that drove me nuts trying figure out why it would overheat, but only did it in slow traffic or long idle periods.
Turned out to be a very small leak in a head gasket which did not affect performance,nothing showed up on the plugs.It ran just fine but got hot.
The problem was found by sticking this device on the radiator fill hole and it could detect the presence of exhaust gases in the cooling system.
Now I'm not saying this is your problem, but I would look at the idea of too much heat cause it sure seems you've done a lot to cool it down and its not hot out either.
Dan
