Wouldn't it be cheaper and easier to just drill the fork to take a recessed nut? You can use the longer front axle on the rear, and the rear brake on the front with a longer recessed nut.
I posted the following in another thread ("brake mounting bolt not long enough?") 11 days ago -
Originally Posted by
stachemaster
Didn't see it mentioned above, but for anyone wanting to run two recessed-nut brakes on a old frame: what mount the new front brake on the rear, making sure to swap the pads around so the pads dont slide out of their cartridges, and fix with a regular nut.
mount the new rear brake on the fork (again, swapping the pags l-r) after drilling out, and use a longer recessed nut (like one for a carbon fork) to hit the bolt. your brakes should've come with several recessed nuts of different lengths.