Hi Sarah and welcome
When I relocated Polly she was in the basket for around 5 hours. Halfway home I stopped the car and let her have the run of the inside of the car for half an hour or so. She was just fine, shared part of my ham sandwich then settled back down happily in her basket.
In your case I'd have some puppy pads to line the bottom of Mollys basket just in case of accident, and stay with her in the car if you're allowed to do so. This should reduce her stress as she won't be aware of the changes going on outside the environment of the car.
Rosella's idea of taking two baskets is genius. I have collapsible carriers which might make sense for this trip. This one looks good and wouldn't take much room.
http://www.petplanet.co.uk/product.asp?dept_id=771&pf_id=60234Zylkene really works, sprinkle on her food for a couple of days beforehand.
Pet remedy spray is also very effective, buy the tiny bottle and see if she responds to it, my lot love it.
http://www.petremedy.co.uk