For what its worth, you might want to reconsider a hitch mount. Both Yakima and Thule have hitch mounts that swing away from the back of the truck so that you can open the rear door.
If you are still hot for the roof mount, just remember when you've got the bikes on top of the H3. Back when I was younger and biking racing ALOT, I saw more than 1 bike and rack assy. laying on the ground, mangled after someone forgot that they had bikes on top and drove into a garage. Oh, by the way, I know someone (NOT ME, thank god!) that did this.
Can you say "...Not Happy..."?