Good thing is most cats like to sleep a lot during the day so unless you're travelling at night I'm not sure why a sedative would be necessary. We've done similar lengths of journey with two cats and no problems or need to drug them. After all the cat will be with you in not entirely unfamiliar surroundings. Here's some tips:
1. Let you're cat do it's stuff before you set off. But for heavens sake don't let it wander off or you may have to wait a long time for it to come back, especially if it thinks you're taking it to the vet. Ours used to run a mile if it caught a glimpse of the carry basket.
2. Put your cat in a decent sized closed carrier. Don't allow your cat freedom to move about the car when you're driving. If you're worried about it stop the car to take a look.
3. Make the carrier warm and familiar with it's own blanket etc. This not only makes it cosy but protects kitty if you have to brake. Put it's favorite toy in if it has one.
4. Take plenty of water and make sure if it's hot and you have no aircon that the car is well ventilated in the back. Challenge is not getting the basket soaked so you probable have to stop every hour to offer a drink out of a dish.
5. If you take a coffee break (and on a six hour plus journey you should) and decide to leave felix in the car on a hot day remember to keep a window open.
6. have a change of bedding to hand in case of "accidents"
Good luck.