To prevent this from occurring, the Builder's Club has created a stencil that lines the holes in the shoulder up in exactly the right position. The shoulder itself must be centered first, of course, but with careful measurements it is fairly easy. Once the alignments had been marked, I was able to begin drilling.
The first items I drilled were not on the frame itself; they were the shoulder holes. While gluing they had been clogged, but a quick run through with a drill quickly rectified that. Next were the shoulder holes: six smaller ones and one larger, central one which would permit the wires access to the foot shells.
After drilling, I tested to ensure that all of the bolts fit properly in their holes and that none of the holes had been at an angle. A few had been at a very slight angle, preventing the bolts from getting to their seatings, but with another quick run of the drill, they all fit.
Next I attached the legs as a further check. With the aforementioned improvements, they fit! It was just a matter of getting the alignments right.