I just ordered some Yakima components from REI (Recreational Equipment
Inc.).  1-800-426-4840.  You don't need to know the part numbers, just
tell them what vehicle you're outfitting and they'll look it up.  For
my '87 Vanagon (no pop-top) I needed the 1A Tower rack plus some
"extenders" to enable the bars to clear the roof.  The recommended
crossbar length is 66", but I went for the 78" bars instead (same price).
This simply gives you more room (eg, 2 canoes) and if it turns out that
its coming close to knocking street signs down, they can always be shortened
with a hacksaw.

As for Thule vs. Yakima, they're both excellent systems.  The Yakima has
round crossbars vs. square ones on the Thule.  On my 58" Yakima rack for
the Jetta, I can barely squeeze 3 bikes and a canoe.  To do this I had to
play with the bike fork mounts by rotating them on the round bars so that
the toys could be "snuggled" together.  I don't believe that this sort
of fine tuned adjustability is possible with the Thule.

Tom Ervolina