If you've described the years and models correctly, neither of those two vehicles would have had a 6-cyl diesel.

But, if it really, truly is a 300D with a manual (either the 115 chassis-up to 1975 or the 123 chassis - up to 1985), then it would almost certainly have to be a Euro spec car. I'm pretty sure that the 115 pedal group doesn't swap to the 123 chassis but, darn near everything else will. So, in that case, I'd pick it up because it will have a lot of the stuff you'd want for a conversion to manual in a 123.

OTOH, if it's a 220D with a manual, I'd probably pass it up. These trannies are stout but that one was never intended to have an OM617 in front of it. Over the years MBZ made a lot of changes to the manual tranny that included things like beefing the 1-2 and rev gears and their associated bearings.

Good luck.

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Never a dull moment at Berry Hill Farm.