I would stick with a Yakima or Thule system.  Strong, versatile,
removable.  I can't really think of any vehicle where the factory
installed racks would be suitable for 100lbs worth of boats - Except
for maybe a Nissan Xterra.

Cheers,

We bought these things called Mighty Mounts made by Yakima from REI to
install a tray type bike rack on the roof of our '93 Previa.  The Mighty
Mounts wrap around the cross bars of the factory rack and the bike tray
mounts to the Mighty Mounts.  I don't know if you can mount the Kayak rack
on the Mighty Mounts or not, and Yakima probably doesn't recommend them
because the kayaks may weigh more than the capacity of the rack. If they are
the short river kayaks you probably wouldn't have a problem but the longer
narrow kayaks may be a problem.  The factory rack is mounted to the roof
with nut-serts, which are basically threaded rivets.  The biggest problem is
lift so if you tie the kayaks down to the front and rear and don't drive
very fast, you may be able to get around the lift problem.

We mounted a clamshell roof-top carrier box on the roof rack of the Previa
for a camping trip, and our highway mileage dropped from 23 or 24 down to 14
mpg.  2 kayaks on the roof will probably give similar results.  We have
since gotten one of the long aerodynamic carriers from Yakima and it makes
little difference in fuel mileage on our Sequoia.

Good luck!

If you're willing to make a rack, you can save a lot of money over the
cost of a Thule or Yakima rack that fits onto your existing Previa
rack.  Given the age of the Previa, that might be a good idea.  Check
out the ideas on Kayak Wiki, especially the first big paragraph.

http://www.kayakforum.com/cgi-sys/cgiwrap/guille/wiki.pl?Roof_Racks

Mike