BITD- when brakes used regular nuts in the back, brakes were held onto forks like yours with the adapter in front and a plain washer in back.
You have a few choices, one of which will make your life much easier.
By skipping the rear adapter spacer, you might gain enough thread to go back to your plain nut. Or you can use a file to make a bit of a spot face in back, and stay with the nut.
Or you can drill both the fork, and back adapter and go with a recessed nut. Or you can drill the fork, and file a spot face. Or use a
step drill that will make the through hole and spot face or counterbore in a single pass.
So, you know what you have, and can make a decision among the various options depending on the tools and skills you can bring to bear.