When you use the 24T pulley conversion, you need the S3M198 belt (66 teeth, 198 mm length) for the rear and S3M384 belt (128 teeth, 384 mm length) for the side when you use with the 25T front side pulley. You can also use the Yokomo ZE-198 rear belt for the rear.
If you use 27T front side pulley, you need another belt as the side belt. I don't know the number of teeth you need. Perhaps somebody else can help...
Using 25T will create front overdrive on the car. It should work with the 24T pulley conversion the same as it does if you're on the stock 19T pulleys. To get back to the same front and rear tire rollout, you need to run a 4 to 5 mm split (front tires 4 to 5 mm smaller than rear).