questions: (1) If I bolt them thru the roof of the
: topper, what tink of backing plate do I need on the inside? and what
: should I use to get a water-tight seal?

Greg,
I bolted a rack through my jeep hardtop with rectangular aluminum plates that were about 3" x 5" and 1/8" thick (two bolts through each plate-I recommend stainless) on the inside of the truck. I was able to shake my truck all around by grabbing on to the cross bars and pulling. As for a watertight seal....I'm sure that any gasket sealer etc from your local hardware store would work great....I never had a problem while I had the mounts bolted through.