Originally Posted by pgoat

Thanks folks

Unless I can find this right angle drill attachment I will probably go with the drill from the outside (rear of bike).

The part of the boss to which the caliper actually rests on is actually squared off - like a little steel box in the center of the round crossbar connecting the seat stays. So even a regular flat washer should work. I am pretty sure I have some of those concave washers laying around just in case (I imagine I am like most on this board who keep all these little parts from many many old bikes ago - never know when they will save the day! )