In the stereo industry, there's a basically standard size for replacement stereos. From what I've seen, there are two places you could locate this. First, up above where the stock stereo resides in your car (also an excellent place for a three gauge cluster), and down below in front of the shifter.
I've seen some people who've installed aftermarket stereos in the upper location and it looks like they've sourced a piece of trim to sit around it in that particular section of the dash. I'm not sure where to get these, so if someone knows, please share.
We're also interested in upgrading the stereo in our Project 245, but I think I'm leaning towards either a unit with a removeable faceplate or one of the new Sirius Satellite Radio units as that's the system Volvo will be going with once it goes live.
They have a huge selection and a vehicle size guide as well. They have trim pieces to make the unit look good in the dash as well. I have purchased from them before and they have excellent customer service and a very good reputation, IMHO.