I agree, I would love to have a turbo 300D with a manual transmission. This would be my ideal 123 car.

The manual from a 240D would mate to the engine okay. There is a difference in the rear of an automatic crankshaft and a manual crankshaft, but I understand that a pilot bearing can be fitted in some way, you would want to research this part thoroughly before proceeding.

Once the transmission is mated, the other problems you will run into will be: driveshaft will be the wrong length, you will probably have to have one fabricated, transmission mount will be in the wrong place, this probably won't be a big problem to overcome, shift rods will be the wrong length, you will have to fabricate new ones.

Every time I have thought about this one, there are a number of people who discourage me from doing such a swap. I may do it someday, however.

Good luck,