We have a GT2835 on our III and it was on the evo 1 we have previously.
We had to have a flange welded so it would fit as they are slightly different (angle I think)
We do not have any problems with ours, but I would suggest you make sure your fueling is ok and that you are not going to run your fuel pump or injectors close to the max. If so I would suggest upgrading them at the same time.
I have just done a google seach on garrett a/r 84 and came up with this link
http://www.aptuning.net/Garrett_Turb...a-fslash-r.htm
Is there a metal plaque on the turbo housing at all?
T