Am I going to throw a belt doing this?

The more belt engagement the better. I would say at least half the pulley coverage minimum. Any less and it could slip. Thats for all the pulleys. Lay a straight edge across the center of the pulley and it should meet belt on both sides. Yeah there will be some custom situations where this is not possible but its not an idea situation. If you cant get it to work out you may need to fab a bracket with an idler.