A 13-5 is closer to a 12-7 than it is to a 12-6. The Irvine 53 will spin a 13-5, to be sure, but it will be a fair bit off its optimum. If you need a 13" prop for it, a 13-4 will have ~ the same engine load as your 12-6, but you may not want to limit the level speed that much. I think that a 12-5 is a good all-around choice for that engine.