----------snip---------------
Sound reasonable?
Nope.
The triple won't likely clamp down that far, and even if it did, it wouldn't clamp evenly, IE the 40mm hole wouldn't be round.
You need to measure the diameter of the steering tube to +/- .001", and give the top triple to your machinist so he can make the OD of the collar to fit.
Ideally, have your machinist measure the steering tube.