Do yourself a favor and either seriously, seriously think about this idea...or don't do it at all. You'll be pretty well stuck with it afterward.
I would go with the latter on that. My dad had a sunroof put into my '84 GMC without my knowledge and the installers did a horrible job. Sharp edges and it leaked for the longest time. Almost bad enough to call it "butchering"...
When I find one, I'm going to change the cab. It is time anyway...
William