gm-8/gm-4 turbo q.

I have a gm-8 turbo off of a 98-2000 gm 6.5L desile on my Teg right now, it has lots of shaft play and leaks so it smokes. I bought a gm-4 off of a guy for $50 and its 100% mint w/ no shaft play at all. Its off of a earlier model gm 6.5L. anyone know the diffrences in the size? also can I put on the exhaust housing off the gm-8 on it so I dont have to have to make a new dowinpipe?
Thanks