I run an 850 estate (as a works van!) and can confirm this is a bitch of a job to do, seeing as the exhaust manifold is right at the rear of the engine and there is loads to remove to do the job. On top of that, the nuts will be well corroded on (and getting hot/cold all the time doesn't help) and there is b**ger all room to do the job anyway.
Have a look at the Volvo Owners club forums (850 section)for more help.
http://www.volvoforums.org.uk/forumdisplay.php ?f=16
Your best bet is to find a local independent Volvo garage not main dealer!) who can do the job for you.