Hi there, I'll start with I have worked with and owned landys my whole life so I am very familiar with both 200 and 300tdi's my query is has anyone started with a fully stripped 300tdi cylinder block and built it up as a 200tdi (disco) engine? I have a series hybrid with a 300 that I want to swap for a 200 for the many reasons we all no but I want to keep the current 300tdi engine mounts towards the rear of the block braced to the flywheel housing but 200tdi blocks don't have the threads to fit them also fitting a 300tdi flywheel housing on 200 block is possible but has no way to fit the crank seal.

I think you've basically answered your own question..... the 300Tdi was designed to be an upgrade of the 200Tdi. As such, aside the differences you've noted with the crank seal, there are significant differences in the lubrication (oil pump) design, block strengthening, ancillary drive, cylinder head, injectors, etc which would make the process of "frankensteining" the two a tad bit difficult.

If its engine mounts you need to adapt, there is the option of fabricating an extender of sorts with the suitable grade and thickness of steel plate, bolted to original mounting points of the 200Tdi.
There's a company online called "tuff mounts" who specialise in these sort of conversion mounts out in Australia - you can check out their website and draw inspiration from some of their designs.
All the best with it.