I can also recommend these guys here
see my review on Larry Chase's page

http://www.roadhaus.com/shops_search_map.php?state_id=3

Search down the list for "German Autotec"

I did not have it converted.  Personally, depending
upon the fundage you have to keep the A/C running
on the beat, I would not convert it.  Better to R&R the
system once over, do it right, and fill it back up with
the R-12 that will make it ice cold.

But then again, this is Phoenix.

Get quotes from Dave at German Autotec and
Perry (the other recommendation on the list)

I've had Perry do work for me as well.. good guy.

Dave at German Autotec specializes in A/C systems
and was very meticulous to make sure everything was
done right.

Depending upon the current state of the components
(i.e. compressor, pressure relief switch, hoses etc) this
could cost a boatload of money.. esp if you refill with
R-12.

 -tim

ps  look at it this way, inket printer fluid is still more
    expensive than R-12 (compared by the oz. ;-)