It's not just as simple as that, my suspicion is, that as it is 1987, it was probably one of the 2.5's petrol or diesel, it could have been a V8 I guess but then it would not have had the LT77 gearbox but I guess it could have been changed too!
So lets assume it was a 2.5 engine with an LT77 gearbox, then the engine would sit fairly far back and the resultant position of the LT77 and the LT230 boxes is dictated by the engine, the prop-shafts therefore then suit this position. If you then move the engine forward with your existing gearbox you end up with the gear-stick in the fuse box!
It is common for the 300tdi to be fitted in the 2.5-200 position when done as a retrofit because it lets you leave the most things alone without modding and changing lots.
In your situation before you go any further you have the option to move it and then use Defender 300tdi parts for everything else, but you would need a Defender R380 gearbox because they never did the LT77 with the longer bellhousing like the Defender R380.
I have a 300tdi in my originally 2.5P Defender and I bought an Ashcroft R380 short shaft because I destroyed the LT77 fairly quickly behind the 300tdi (I knew it was coming but I just ran it till it died).
I would now like to sell the R380 short (less than 20,000miles on it) and get a Defender R380 or convert my short-shaft to a Defender box and basically make it as a stock 300tdi would be because I am not happy with the air filter arrangement and I would like stock hoses and things too (really I am just itching for a Defender project!).