its actually alot eaiser than were all making it seem, You all need to talk to CharlieF1......... He has pretty much gotten the swapping part of the T3 assembly down.
But here is what I can offer. things can be swapped around quite easily if you keep parts together.
An 86/87 compressor housing can be swapped onto the 84/85 cartridge as there all T3's the catch is again the backing plate and the seal. As long as it it T3 based any wheel/compressor/turbine/cartridge can be built. The key is that it is balanced after you mix and match. A place called GpopShop, does my balancing for 55$ and turn around is resonable. Great people to work with.
Now, you can get a carbon seal for the 86/87 backing plate... it only requires a 3 tang carbon retrofit seal... most rebuild kits offer the option. Again, Charlie set me on the way for this.
I myself will be using a converted 86/87 turbo for my carb turbo system as it offers the better turbine wheel - allowing for much much faster spool compared to the 79-83 and 83/84 turbos.
A.j.