Thank you for the responses.
Goody's Automotive in Scottsdale set the ignition timing as well as the air/fuel mixture. They appear to be the GO TO shop for the 450SL in Arizona, so I have to assume those factors are in check.
The condenser fins are straight and so are the radiator fins.
Motor oil is 0W40. I'll check if it coasts well to determine if there is excessive drag.
I am not an engine mechanic but I think that the issue may not be a lack of cooling but one of generating more heat that what the cooling system is meant to handle (if that makes sense). Not sure if this relevant of not, but if I open the hood when the engine is hot and running, the exhaust manifolds read mid 400's F (cylinder 4 reads mid 500's).
Goody's mentioned that 212 F was "PERFECT" and "Normal" but now that we have triple digit days in AZ I find the temp readings of 220/225 ALARMINGLY high.
I just don't want to make the mistake I made earlier of replacing good parts blindly.
Measuring temperatures with IR remotes is handy for checking radiators for cold (blocked) spots.
Drilling a few small holes through the thermostat plate was quite common for hot climates when I lived in Africa and later the Middle East.
Old Mercedes diesel hands should be familiar with it too.
Re de oiling, I used to see detergent in Porsche shops close to 50 years ago. A separate citric acid flush should be done before new cooling system parts (in contact with coolant) are installed. Otherwise, if a water pump is getting close to the end of its cycle, the citric acid flush could be the kiss of death. The bare post C A flush minimum required would be a new thermostat imo.
Anyone who ever removed the heads, has seen how full of gunk the coolant channels there and in the block are.
The blocks coolant channels end at the head.
My friend and oldtime Mercedes mechanic Jerry (RIP) told me, for hot climates the old Mercedes hands would continue the blocks coolant channels by slightly drilling into the heads.
Your coolant system was pressure tested, and held pressure. It would be interesting to know the pressure measured, and whether it was within Mercedes spec operating pressure?
The fan coupling swap to 380 sound worth a try. No self service wrecking yards in your area to get a cheap working one to try before ordering more new parts?
If radiator, water pump. thermostat, hoses (incl those behind the firewall) and clamps are good (no leaks) it shouldn't be that difficult a problem to solve.
I seem to recall a AZ or S Texas member having a higher cooling capacity radiator made at a quite reasonable price, but he installed a electric fan as well.