I have half done this recently 9 just the fronts so far and was not too hard. I used the mark4 subframes on the seat. I ground down the outter wheels to fit in the runners and cut the top half of the seatr mount on the floor of the mk3 golf. Welded a plate on the front of this half cut mount and drilled two holes in it. The mk4 sub frames the slid in and bolted up. That might sound confusing but it wasnt too hard. I can see the door cards being awkward so i am getting mk3 leather ones, And the rear seats look like they need som adapting, but im not too sure how much. Sorry if that isnt much help but its all i can offer.
