Thanks for the advice David, it`s made what I was thinking clearer.Dave,
Yes, Bill has explained it. From a production point of view Vincent designed the wheels so the E80 nuts kept the wheel together as one piece. The E80 nuts snug up the bearings and the axle being tightened is just piling on, so to speak, as it is doing the same. When the racers were designed Vincent needed to make the backing plates thicker and lighter. They could make the plate the entire thickness of the old braking plate thickness plus the E80 nut. This could be done because they could use the axle being tightened in lieu of the E80 nuts.
This is the same design I am suggesting for the caliper plates. You have the the backing plate thickness and the E80 nut thickness to use for your caliper mounting plate. The fork won't notice a thing.
It may be a little over-kill, but you should use the size brakes from a modern machine that is hauling down a 500 lbs. motorcycle for a twin, or a 420 lbs modern bike for the Comet. That is the easiest way to size them.
I`ve said before that the Vincent is proving a steep learning curve for me.