When you replaced the standard fan and fan clutch
 

with electric fan/s, what did you do re the aux fan switch in the top front of the engine? The aux fan is in front of the radiator. Aside from its use with AC, it is my understanding that it also comes on when heat rises from load conditions. You may have replaced its functions with another electric fan, but the aux fan really reduces the temp when it comes on going up hills. And, if your new fan relies on that switch to turn on, you could have a bad switch which is very common. Also your definition of normal temp may be quite low. I have read that anything below ~110 is OK-but only under load.

We are at about 3000', yes the air is a tad thinner, but I don't think it matters. It increases the boiling point of water a few degrees.

I haven't seen many sdl up here, if I see yours I'll give a hello.

I've got a 16" fan and a 10" in a steel shroud that is 1/2" above the radiator. They both kick on at 80C. My controller uses the MBZ temp sensor for its reference. The temp I read on my gauge matches my IR gun within a few degrees.
It seems worse since I installed the new radiator and thermostat, but, the ambient temperature has risen lately. Last few days its been 90-100F. AC sure works good...too bad it exacerbates the temp issue:(
Anyhow, once I get my home projects under controll and the stock market takes the dive, I'm betting on, I'll have the time and a few extra bucks to devote to Whitey.

After reading the troubleshooting guide, I'll investigate the air flow through the radiator. Although, at freeway speeds the fans are not the main source of cooling.

You've clearly replaced every single component in that system and, at this point, it should be operating perfectly.........with one glaring exception.

When you change the airflow characteristics via the use of different fans/shrouding........etc, the result must be considered an experiment. You claim that "plenty of hot air is being emitted by the fans", however the engine might beg to differ with you.

Airflow across the radiator is a complicated subject at higher speeds and the fans may impede this airflow rather than enhance it.

I'd change back to the stock setup and see if your issue disappears.

Yes, that makes perfect sense.

I installed the original mechanical fan. The overheating is worse. And the engine feels more sluggish under acceleration. So, I went back to my electric fans. Also, today I removed the new MBZ thermostat and put my original MBZ thermostat back in. It runs cooler. About 10-15 degrees C cooler. Both thermostats have identical P/Ns and were bought from the same MBZ EDH dealership.

And now I measure 30-40 degrees F difference between upper and lower radiadtor hose.

I am happy with the reduction in operating temperature, I am, however, flummoxed by the difference by changing 2 identical part number thermostats. I have other projects that compel me, so I can't delve into a more definite description of my overheating problem.

I ended up replacing my fan clutch and drilling a hole in my therm and all is nice and cool. Just a hair over 80C .

Fan clutch tests good.
What size hole did you drill in the thermostat?

Clearly, M/B quality control is not what it should be............

Update
 

When I went back to my original thermostat, the overall engine temps did drop, I still was still unable to use the ac going up Bass Lake grade on highway 50. If ac on then engine temp would hit 110C, with it off, about 95-100C.

Today I modified my spare thermostat by cutting the springs out, heating it until it was opened 1/2 inch, then soldering the main brass shaft and the diverter in place. It fixes it open all the time and retains the diverter.

It was cool today at 80F, I drove up Bass Lake grade with ac on at 65-70mph and the temp hit 95C max. I am a happy camper. I know that I'll have to change back the original thermostat in November, but its a small price for the engine temp reduction.

Before, when I re-installed the old thermostat, and today when I installed the modified thermostat, I had to drain coolant. When I poured the coolant back in, I poured it into the upper radiator hose untill it started to dribble out of the radiator top hose nipple, the rest of the coolant was poured into the tank. This method seems to get more air out of the head. Later I parked the car on a hill to add coolant to the hose and found I could not add any, usually I could add coolant a couple of times before it was full. This may have been what dropped my engine temps when I re-installed the original thermostat, unable to say for sure.