The 5/8" radius is the same as a 1 1/4" diameter. The point being made here is you can't put a 5/8" radius on the outside of the cowl cheek and square it off so that a ball with the same diameter (1 1/4") would hang out of the cowl area. If you look at the build thread of my new design "Invictus" you will see what I did. (
A little secret here, if you take a piece of 1 1/4" PVC pipe and cut it into 1/3's or 1/4's, you can put sandpaper on the inside and use it for a 1 1/4" guide 
) As long as a ball or sphere that size would not be exposed, you should be fine. I would suggest you email Jim Allen (
the head of the new design approval committee) and send him some sketches to get his input.
The Kelly has the cheeks the same size on both sides, so your model must also have both the same size too.
Good looking engine!