Fueling during a run can be difficult logistically, so I would recommend taking two approaches:
1. Train using food typically found at ultra aid stations, like candy, chips, boiled potatoes, cookies, soda, peanut butter and jelly sandwiches, or any specific food a goal race advertises. This will allow you to use aid stations for fueling if necessary. I prefer to use my own food, but have run into situations when it wasn't available. Learning to fuel off what is available can solve a lot of potential problems.
2. Find foods that work well for you. This includes foods that are easy to carry, can be stuffed in drop bags, or can be carried by your crew. Since many ultras are run far from civilization, non-perishable foods that require little or no preparation would be ideal. Also, your palate will change throughout a race. What tastes good at mile five probably won't taste good at mile 35 or mile 95. Test different foods at different distances.