If I am reading correctly, you are producing 600 (1/4") slices for service. One person, slicing by hand, should be able to complete the task in 20 minutes working at a moderate pace. My suggestion would be to roast and slice on the day of service. Start slicing about 30 minutes before you begin plating. As you go, place slices in a hotel pan, cover with cellophane and foil and pop into a warming oven or a cambro. When plating time comes, you should be good to go.