Alan:
I did not use a VW body-to-pan rubber gasket on my car.
I only used GE Commercial Clear Silicone Sealant from Home Depot's infamous Paint Department. It comes in "normal" tubes for home use, as well as those honkin-big tubes for pneumatic guns. I borrowed a pneumatic caulking gun from a friend and laid down two 3/8"-1/2" wide beads side-by-side along the pan-to-body joint (it took three big tubes for the car). I used the pneumatic gun to get everything done before the caulk fully set up, but I found that it sets up over about 15 - 20 minutes so a hand-worked "home" gun would have worked just fine, if you're quick.
After I went around the whole joint, three of us lowered the body down onto the pan and wiggled it around until the four, corner bolt holes lined up (dropped a bolt in each one), and the wiggling smooshed the caulk around and filled the voids. Still, I had a few gaps at the rear, and a BIG one at the top of the tunnel at the front, which I eventually filled with successive layers of clear caulk, smooshing it into the gaps with a finger. I did that to any other holes or gaps I found, too (non-joint related, like access holes and such) as I was going around the pan.
Wiped away all the excess inside and out to make it all neat and let it set up while I was putting all of the body bolts in, then tightened everything down and trimmed the inside/outside joints again and that was it. Probably'll adhere forever, and as far as I know I have no leaks.
When I've been caught in the rain while driving and could keep moving, the floors never get wet so nothing's finding it's way in through the joint.
gn 8>)