Eight years ago I did one of these on a 1980 300D. I removed the manifolds and used a piece of 3/4 hex stock, supplied by a neighbor that works at a machine shop.
The hex was Long enough to fit in the plug and leave enough exposed to be gripped by a 3/4 impact socket. The plug finally came loose with a 5-foot length of pipe on a 1/2" breaker-bar, without using a torch. I might not have attempted it had I heard any horror-stories about the possibility of cracking the block, but this forum wasn't around then.

Happy Motoring, Mark

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