I'd be a tiny bit concerned about the brake arm not bottoming out on the base of the boss. But the boss not continuing all the way through the brake arm wouldn't bother me at all.
As it's a brake arm with an integral bushing, the pinch from the torque of the screw/bolt holding the brake arm should effectively block out any bending of the boss. (there'll be some bending stress at the base, but almost none along the length of the boss.
But I would research why it won't slide on a bit more. Is there a wider recess at the fork/frame side of the brake arm bushing that won't quite fit the wider base of the boss? Could the base of the boss be filed down(made thinner) a tiny bit to allow for a better fit?