It sounds like you've covered your bases fairly well - is the fan shroud intact and installed correctly? If it is, then the problem is just too much load and not enough speed to carry the heat out of the radiator.

Just to be clear, is your coolant mixture 70% water/30% coolant or 5-1 ratio as mentioned in the other post? 5-1 (roughly 85%/15%) will dissipate more heat, but either way, the more water in the mixture the better your radiator will transfer heat.

Also, keep in mind that it's normal for an vehicle to run hotter when it's under load like that. It's within a safe operating range unless it's approaching the danger zone on the gauge. Even 3/4 of the way up the gauge would be normal on a slow uphill climb.

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.

Got back from our trip to a little town called Pai about 130K away - located in a valley but you have to cross a rim of mountains to get there - 762 turns in the road (and we have the shirts to prove it - if come up the other way it's 1500 or so turns) .....uphill grade going in is not as bad as some others but there are a lot of hairpin (345') turns that knock the speed to really slow - worse if following a lumbering truck/bus and back in line.....on the way up did OK - never did go above 1/2 gauge - usually runs 1/4......got to our resort and checked in - went out to check the level......the coolant tank was where it wa scold before leaving - thought = all OK.....went out motoring looking for a waterfall we never found.....long higher grade and it started climbing on the gauge quickly.....no waterfall - turned around and made the day of it wround town - all OK......checked coolant bottle leve - seemed the same.....headed home - the climb up is steeper and slower going back....thank goodness for car sickness (wife).....stopped on a downhill while & she recovered for a few moments checked the coolant level - OK in bottle - but NO pressure at the cap (within 6 months)......popped the cap and added 1 liter coolant and 1 liter of mineral water......bottle warm to the touch but level looked same - WTH? - ran on a little longer (3 valleys with surrounding hills) and she went green while the temp gauge stayed OK ..... NO pressure at the hose - cracked the cap and checked - added another liter of mineral water......no change in coolant level at the bottle - looked closer and the bottle is wet on top - as well as the cap wet on top.....the hose was loose enough that I moved slightly and could pull off easily & wet inside (bypassing).....today - clamped the hose/put a 1.1 cap on along with oil change and crankcase oil catch hooked up.....heading out on 600K+ trip tomorrow and will see how it goes - this trip however while having steep grades is a 4 - 6 lane highway where the speed can be maintained and it's not been a problem.....that's where we are at at the moment.....

The only thing I have seen deliver such persistent overheating is a warped head. These engines do like to overheat due to borderline power and gearing, but with all you have done it should be running cool as a block of ice.