Bizzare subject for the newbie to have to chime in on, but:
Forget the RWD 6 cyl 2.4, it was a PITA for head sealing and cracking, and is way too long. However, if your REALLY want a big-engined Golf diesel, consider the 2.5 TDI 5 banger. It WILL fit (barely), but you must use the European Volvo FWD gearbox (from this same engine). So, the way to do it is to find a complete donor Volvo TDI in Europe and have someone send over everything that isn't a body panel or interior part. The engine, is, of course, 100% VWAG.