Well, can't really help you there; don't know of anyone who's done it.
But, taller gearing on the 300SE would surely be welcome.
I toyed around with the idea of installing a taller differential, then tweaking the existing tranny to always start in 1st gear - effectively getting the same results as installing an OD tranny. But, I haven't ever pulled the trigger after 20 years, so it likely won't happen.
I would assume, to install a later OD transmission, you would face all the problems (and more) that we do in Chevys and Fords: the bellhousing wouldn't mate to the engine, the starter wouldn't fit, the flywheel wouldn't bolt to the tranny, the driveshaft would require a length change, the later tranny would be computer controlled and require major electrical tweaking and interface with the engine controls to behave properly, clearance under the body would rear it's ugly head, tranny mount wouldn't adapt.
And, if nobody's done it, then you'd get to learn all these little tidbits on your on. I'd say if you can't find info from a previous successful job, look closer at the option I mentioned above.

But, I'll hide and watch with keen interest; keep us posted.

DG