Adding many things to water raises the colligative boiling point and lowers the colligative freezing point, but also lowers the specific heat and thus the heat transferring ability. You only want enough antifreeze to provide corrosion resistance. Are you downshifting on the grades so that rpms are in the upper part of the range (don't know the redline on your engine)? The faster the engine is turning, the more air the fan pulls thru the radiator, and the more cooling you get. There is a grade in N. GA that would overheat my turbo swb, on a steep forest service road in the Coweeta hydrologic lab, when I pulled it in D hirange. No problems if I shifted to 4lo, higher rpms and half the load.

If you have manual hubs, unlock the fronts and put it in 4lo and scurry up. If no manual hubs, put it in the lowest gear compatible with redline.