I don't even bother to try adjusting (loosening, then retightening) the mount of the A/C compressor because, in contrast to the P/S pump and Alternator which have easy-to-adjust mounts, it's almost impossible to reach all the nuts to move the A/C compressor.

Instead, I just take a big screwdriver, and a long-handled ratchet with a socket to fit the crank pulley's central bolt, and (with the new belt fully on the A/C pulley) I turn the crank pulley while prying the new belt over the lip of the crank pulley. It takes about 15 seconds. Like already stated, it's the outermost belt, so it's really easy.

As for what belt to buy, I have two '93s, which (although they have a different A/C compressor than yours) ought to have the same belt as yours since they're all B230s. So with that caveat (they may be different than yours, I can't be sure), I'll offer that my belts for both of my cars is size 12.5W x 935L
Volvo (should you want to buy it at a Volvo dealer) shows two p/nos that show that belt size, either p/n 979277-1 for $16.89 (list), or else p/n 973489-8.

Good luck.