its the beam itself that is wider, but the mounting points are the same, as are the stub axles. however not all mk3 beams are the same width, its a bit confusing! there is a large thread on clubgti with various beam width comparisons. I might be wrong on this but I think the passat beam is wider at the mounting poits, so it wont fit a MK2. Perhaps

you can convert the rear beams between 4 and 5 stud simply by swapping the rear discs, so either way its no bother. Personally I'd stick with wheel spacers or stub axle spacers, rather than all the effort of doing a beam swap. unless the car has no rear arb or rear discs, then it would be more worthwhile to go to all the effort

edit: doh ben posted the link to the beam faq, teach me not to click before posting
