I picked mine up from an auto jumble for 5 pounds

, It wasn't a volvo 340 one but with a little fabrication it fitted, makes a nice sound and the car feels a tad different to drive (little better throttle response i think) but doesn't really do anything in the speed department, and i wouldn't of wanted to spend too much on one.
Someone will probably correct me on this, but seeing as the 1.7 has a tetchy carb as standard, you may save your self some problems by sticking with the standard air box.
And ive never had my carb ice up on me lol so i dont think that is too much to worry about but mines a b14 and it may be different to the b172's in that department.
Dan.