Charlie,

The '86 SDL and later models had a completely new 6 cylinder engine unlike the 300D which was a 240D with one more cylinder. Thus very few of the parts are interchangable. Even the oil, air and fuel filters are different.

A 5 cylinder cast iron (not steel) will never fit a 6 cylinder engine. I also wish it was possible to get a cast iron head for a 300 SDL. I can never understand how an aluminum head works on a cast iron block because of the difference of the thermal coefficient of expansion. Aluminum expands about 3 times as much as cast iron.
So when the engine heats and cools, there has to be a sliding motion between the head and the block. It seems to me that this sliding action would cause the head gasket to wear and leak.

P E H