Advanced Turbo Questions

[cjburn_2 at yahoo.com]

I've always been a lurker on the list and post more on
the forum but if anyone can help me out here, I'd
really appreciate it:
I really don't understand the A/R concept too much. If
the A/R ratio describes an area ratio relative to the
opening of the turbo to the general area inside the
housing, how is it possible to have A/R change with
wheel diameter? Does this happen? I'm guessing it does
since there are different part numbers for different
housings given for a different wheel. Lastly, has
anyone ever mounted a T04S housing in the front of an
SR, being mounted on a GTi-R manifold? Is it possible
with enough grinding on the block? Or would I be
better off getting a T04E housing cut for the wheel
I'm going to be using. Nobody who runs a business who
sells turbos has a clue to what I'm trying to get to.
It's absolutely hilarious.  Thanks in advance....