Mine was built/configured that way when I bought it.
Mine is also a 1986, but was compleetly restored in 2008 by Liveridge UK.
300TDI, R380 and RRC transfer case 1.2 gearing.
Great set up.
There is a few threads on here that may help you find a source for a 300TDI. Otherwise, I wouldn't know where to start looking.
Good luck ! it's an awesome setup, you should definately do it.