I have adjusted the valves many times on my two 300D's. I never used special wrenches. One from my Craftsman set and one from a set of thin flat ones I bought at Harbor Freight. Both are 14 mm, I recall, which 9/16" will also fit if you have an SAE set. Of course, it is much easier if the injection tubes are off, so plan a valve adjustment the same time you are pop-testing injectors, and also a good time to change bad glow plugs.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans