We sell a accessory pocket that will snap in above or below the new radio you are installing. Once it is in place install the new radios sleeve to hold the radio to the dash.  If you are using one of our Volvo harnesses simply plug into the trucks truck wiring and you are good to go.  Here is a link to the Volvo pocket.
http://www.bigrigstereo.com/volvo_white_gmc.html
You can use a double-din but there are no aftermarket version that can be classified as Heavy Duty