I agree dropping one more gear and running part throttle and keeping the RPM up is the best plan if the temp gauge is rising uncomfortably. Generally I recommend RPM between peak torque and peak HP when pulling max.
For an older truck it might be worth while to replace the radiator. And if the TC does not lock in second and this is an option to fix with a chip I would go for that. Just don't use the chip to make more power as that can only make things worse. I don't recommend anything that would defeat emission controls.
