You're not likely to find a Volvo head unit with CD player which will fit the single-din sized space you have to work with (all I've seen, like the SC815 or SC 901, are double-din size).
However, there are several single-din Volvo radios which can control the external alpine/ volvo cd changers: e.g., If you want an external amp, the CR-814 runs two channels from the head unit and up to four from an external amp. If you want the amp in the head unit, the CR-915 runs all 6 channels from the head unit without any external amp. Seen both on Ebay for less than $40.
You can put the CD changer in the trunk, glove box, or under the seat and a pocket into your extra dash space.
Lots of info on the bboard, but you may have to try several different searches.
--
We have met the enemy and they is us. [Pogo] S70 cop car : Rough Rider tires& suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps