If you measure that way you need to make sure your drives are straight. You could also measure from the center crank bolt to center crank bolt..
Nothing wrong with CMI's products, but there are also other options out there as well..
If you are running stock or close to stock 496's and you have tight centerlines that are a concern, Stainless marine's manifolds are not really any wider than the stock merc manifolds... We offer those & others.. Feel free to give me a shout this week if it interests you further.. I can discuss them and or some other options with you then.. Jamie / Lakeside 570-639-2628