At the risk of starting a holy war, I'll say that not having MB coolant in your '82 MB isn't your problem. I would ensure that the concentration of coolant is no more than 50%. These engines actually run cooler with < 50% coolant concentration. I run approx. 40% coolant - 60% water and live in a hot climate.
Water Wetter - visit their WEB site and you'll learn this product only helps when a VERY low % of coolant is used.
Ensure you have sufficient air flow; water flow.
Here's a pointer to an old thread that covers the overheating on a hill issue. In the end, the orig. poster solved the problem by replacing a worn out fan clutch.
http://www.peachparts.com/shopforum/showthread.php?t=102579&highlight=overheats+hills
Some believe that the fan clutch only serves to cool at lower speeds and at idle. Years ago I battled an overheating issue on another car. It only ran hot on the highway. I replaced everything there was to replace, then had a conversation with a veteran tech about the fan clutch. Sure enough, it turned out to be a tired fan clutch.
I'm not saying this is your problem. Just something else to consider.