John,

Yes, the upper left grooved idler pulley is the one that tends to go.

Here's an official SAAB document that shows the belt routing.
http://townsendimports.com/Web/engine_folder/ng900beltsc405.htm

Also, here's one from the place where I do all of my online parts
purchasing:
http://www.eeuroparts.com/searchresults.aspx?&carid=1072&cat=1004

eEuroparts is almost always the cheapest and has been extremely reliable
and fast at shipping.

There is no need for a wood block.  Once you flex the tensioner assembly
(with a long 1/2" socket extension or a breaker bar turned sideways)
there are two holes that will line up that allow you to stick a drill
bit or allen wrench thru them to hold it in the "cocked" position.

This is a really easy job.

First, here's a page with the belt route - stock is the diagram on the
LEFT: http://pages.infinit.net/mlbriand/saab/shortbelt/page1.html 

The upper idler pulley that goes bad is the one shown at the top
integrated with the tensioner. The break up after 50K miles - so next
time replace it around 45K. It *is* worth checking the other pulleys
for play as the water pumps in these wear and occasionally an A/C
compressor goes. It is also possible that the lower idler pulley is
bad (center of engine, bottom of left photo) but generally they last
quite a while. To check the pulleys/accessories, just rock them with
your hands. You'll be able to feel significant play in anything bad.

(Lots of us do the "short belt mod" shown on the right, I'll get back
to that).

The instructions are the same for short or long (stock) belts. Click
through the photos. Loosen the tension on the belt using a 1/2" drive
as shown to pull the tensioner forward and lock it into place with a
drill bit or thin screwdriver in the "lock it"  hole. Unbolt the upper
pulley, replace that pulley, pull the belt around the other pulleys
and accessories in the proper route.

You may or may not have the Mass Air Valve shown depending on your
motor but pull out the air filter box to get some room to work either
way. You should remove the plastic cover behind the front right wheel
(no need to remove wheel unless you want to, just turn it hard right).
The only reason you need to remove the cover is that it is hard to get
the belt around the crankshaft pulley without doing it. It's still
tight/annoying getting the belt down there but stick with it, it can
be done.

Once you get the belt right and the upper pulley replaced, just pull
the tensioner forward with the 1/2" drive again to remove the tension
from your "lock" bit, pull the bit out, and release the tensioner into
position.

The first time you do this job it will probably take you an hour but
the second time you can do it in 15 mins.

Las, but not least, many of us do the "short belt mod". In later
years, Saab eliminated the center pulley (lower idler pulley) on the
motor and used a shorter belt. Since the center pulley is a RPITA to
replace, many owners of the earlier design just go to the belt
mentioned at this web site. That eliminates one point of failure and
saves time and money. (Yes, it is a safe mod, many many guys who drive
their motors hard have done it and it works fine.)