From the center of the inner stud to the edge of the outer bore it is 1.41".
Also take into consideration the stud and nut size for both (5/8-18 and M22 x1.5), and the OD of the socket you'll use on the inner nuts.
I agree with your math, and this ~1.41" is as large as the flange on the inner nut can be. With a normal flanged wheel nut this will interrupt the pilot portion for the larger wheel. Having an interrupted pilot diameter is not a problem, but having the flange of the nut interfering with the larger wheels bore diameter is.
You can probably use normal nuts that are about 1-1/16" on the flats, or just under 1.25" on the corners? I expect this will work. You may not be able to remove the adapter once the larger wheels are in place, a socket may not fit because of the larger wheel.
Grigg