404 and 301 responses

[ThatsIT.net.au]

I have a server running iis6 and ISA server for my firewall, I host a large
number of sites
I recently read that Google penalizes you for having multiple domain names
and that www and no www are counted as 2 domain names.
So I though I would fix the problem, Google likes to get a 301 redirect so I
set a up anew web site and using my ISA server I send all requests that are
using the domains I want search engines to use to the correct web sites. I
sent all non www request and all duplicate domain names to the new site and
set up a custom 404 page in IIS.
On the 404 page I get the page info using Request.Url and send the request
to the right page using a 301 redirect. alls fine so far. But some requests
for aspx pages using query strings are handed to asp.net and go to the
asp.net 404 page and not the IIS 404 page. so I set up a custom 404 page for
asp.net. Here is where my problems started. In the asp.net 404 custom page I
am sent a query string like this ?aspxerrorpath=/folder/filename.aspx
with this I have no idea what site it came from and I have lost the query
string.

Is there a way I can get this info, or is there a way I can sent all 404s to
the IIS custom page?

I don't want to set up custom pages on every site this would let me know
what site the request was for but would be very time consuming and still
would not get me the query string.

any ideas?