I see where you are coming from with respect to staying within Humber's weight limit but for what it would cost to upgrade from your 60 it would be expensive for an extra 10 horses.
That One design does look very low in the water, I wonder how different your destroyer would look if you chopped off the raised splash plates down to the same level as the centre of the transom like the one design? I also reckon that the tubes on the one design hull are mounted slightly lower on the hull relative to the centre of the transom?
My Aframe is a double one and I was very surprised at how light it was when I fitted it. It bolts to the inside of the transom and also to the floor a foot or so forward so I hope it provides more strength to the transom than its weight takes away.
I'd love to see how yours handles with that little weight on the transom, I bet you can really throw it around. Let me know if ever you decide to do some ribbing in Cornwall!