One thing I haven't tried yet, but I have read that someone else has done recently. Add in a small radiator, like a motorcycle radiator, and an electric fan attached to it. You plumb this into series with the heater core. Flip the switch to the electric fan when engine coolant temps get to high.

I asked the guy exactly where he mounted it. I think we've all done the turn the heater on and fan to high when our vehicle starts over heating, this is the same theory except this should cool better since it's not in that box.

The guy that did this said it resolved his running hot problems.