It could still be the head or gasket. To check for a cracked head, pressurize the coolant system and see how it holds over night. Be careful on starting the next day as it could be hydrolocked from a cracked head. If it leaks or locks, the head should be scoped out. I got a crate engine from MB dealer with 4 year/40K warrantee but it was expensive. Metric Motors has rebuilt engines for a little less. Smoke could also be caused by IP or injectors if they are the original with all those miles. It could also be timing chain stretch. Also has the ALDA been adjusted?

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87 300D 265Kmi
Factory rebuilt crate 603.96x engine at 200K