Dana:
I purchased a 100 sq ft bulk pack of some stuff called FatMat and had
enough to do my 91 Multivan's engine deck (including covering the lid),
under the bench seat, the front seat podiums (in and out), the entire front
cabin floor, both front doors and the slider.  I used the foil bubble stuff
from Home Depot to insulate the rear passenger area side panels, and the
rear door.
I'm blown away at how quiet the van is now.  I would urge you to install
against the inside of the exterior skin instead of inside the panels.  And
you may want to consider doing over and around the front seats.  Deadening
the road noise from the wheel well makes a big diff.

I have some pics of the project out on my site if you're interested or
bored.

Brent
www.weidefamily.net/vanagon/
go to Interior / Insulation