The safest way is the water outlet elbow/thermostat housing. I have seen disco 200's fitted with serp belts. Global had one for sale @ one point. 200's were made from 89-92.
There is a wealth of information on this out there and google will probably be your best bet. There are very few differences and a lot of what you will read will be personal preferences based on someone owning a strong runner that went forever. The heads are slightly different castings but the manifolds etc are interchangable. The blocks are both 2.49 liters and again pretty much the same. The 200's had two different configurations the disco and defender versions these used different front covers, ancillary's, and manifolds and turbos, again this stuff can be swapped back and forth. The 300 engines had no differences between the disco and the defender except that very late disco 300's had a "fly by wire" system that made slightly more power for trucks with automatics. These can be retro fitted with mechanical injection pumps but there are so many of the mechanical's out there that makes little sense. The 300 is the later design although only by four years. A/C retro install kits are still available for the 300.
Many swear the 200 gets better mileage.
I have too heavy a foot to weigh in either way.