check a pet store or specialty online retailer for things that will calm a cat (try petfooddirect.com). they usually have something you can sprinkle on their food or put in their water that will naturally calm them down. catnip is a good idea, too. while most cats get nuts with catnip at first, it makes them sleepy after a bit. it's a treat for them, too. also, build kitty up to it- take him/her on short trips, first, see how kitty responds to that.
make sure you have a sort of large kitty carrier and get some blankets or something that has kitty's smell on it so they'll be around familiar things in the carrier. the pet store should have these pads for training puppies- it's like a diaper only it's a flat sheet- you can put that in the carrier in case kitty has an accident, then it'll be really easy and hassle-free to clean up.
see if you can't get kitty into a harness that you can clip a lead onto so that if you do decide to travel with him/her you can take him/her out for a stretch at rest stops. hope this helps you!