I think most bike racks would get in the way of opening and shutting the rear door, though. And I'm travelin' on a budget...cheap rope is *vey* cheap. ;-)
I'm probably worrying needlessly about the cats as I've said...it's not like they've never traveled before. Tam used to ride with me every single weekend out to my parents', and Halvah was fine during the 2000 mile trip to CO.
I go along with Ruthy's suggestion of letting them out for rest stops every so often.
Are you planning on staying at hotels along the way or trying to sleep in the car?