I don't think I would let a cat that doesn't know me outside on a trip like that - harness or no. I would do as suggested by others:
check with the vet, use the crate in the car and use the larger crate and litter pan in the hotel room. For a 750 mile trip you are really only talking about one night. I would split the travel evenly so neither day is excessively long for him. And happy pills couldn't hurt, either.
If you do decide to use a harness, here's a handy link, and plan to give him an opportunity to get used to the harness before you start the trip.
http://www.petproductadvisor.com/buy...150/page1.aspx