You may get confusing advice as there are two different methods of fitting a 300tdi into a 90/110.
Firstly the 300tdi engine and its ancillaries are the same for the Defender and the Discovery and Land Rover fitted them in the same position relative to the front axle on both models. (The front of a Defender chassis is virtually identical to a tdi Discovery.)
Assuming that your Land Rover is an early 4cylinder model you will need to weld on new engine mounts, the position of these mounts is dictated by the gearbox that you use.
If you use the Discovery R380 'box then you need to copy the Discovery engine mounts exactly, this will put your 300tdi engine where Land Rover put their 300tdi. The benefit of this is that you can use all standard 300tdi radiator/intercooler/oil cooler/PAS pipes and a standard 300tdi exhaust pipe.
You will also need the front prop off the 300tdi Discovery and the Discovery gearbox crossmember. Your transfer box will now be 2 inches further forward, so you will need to modify the front of the seat box and source a Defender 300tdi rear prop. As others have said, the Discovery gear lever will want to come up around the front of the centre seat.
If you retain the LT77 gear box, the 300tdi engine sits exactly where the old 4-cyl engine was and requires modified engine mounts as the engine is 6 inches further back than where Land Rover fitted their 300tdi's. This also means that you have to extend the radiator/inter-cooler/oil cooler/PAS pipes and a chop about a 300tdi exhaust pipe. You don't however need to mess with prop-shafts or the seat box. Neither do you need to replace the gearbox cross-member.
Whichever option you take, fitting the Discovery transfer box will give you Discovery gearing, not the R380 main box.
Have a look at this guy's site, loads of good info.
http://www.glencoyne.co.uk/tdi300.htm