Guys, changing the plastics is possible and there is no difference in the mounting points.
I've changed all my bodywork on my 1990 XT600E to the 2002-2003 Deep Purp.Blue and it all fits in the right places! even the pillion seatgrabs fit right!
BUT, you will have to change the fuel tank, the seatpost, left and right plastics and the rear fender alltogether as they aren't compatible with the pre-96 models since they changed the bodywork!!
As for the exhaust not being part of the frame structure after 1995 model you are right. but, the exhausts for those newer models will fit on the pre 1996 models
BUT I suggest that you make a bracket of some sort to reinforce the frame from the exhaust mounting point to somewhere else in the upper frame of even the exhaust itself although I think I remember someone saying to me sometime ago they've done this without that bracket I mentioned...Was it you Wolfzero?

As you can see, There appears to be a slight difference between the seatpost and the right side fairing....
It was due to the fact that I welded a new rear shock mounting point to the frame as you can see in the next picture.
The solution for me was to cut a small piece of plastic from the under part of the seatpost that in normal circunstances fit on the frame as it should!
For me, I cut it and made two hexagonal holes to fit some nuts and used two screws to lock the seatpost!
You guys don't need to do this!
As for the ACERBIS fuel tank, I don-t see why it won-t fit as long as you change the seatpost and bodywork as well.
Vando