Any good Chandler will stock a selection of suitable Stainless Steel Drop-Nose Pins that will do the job. (Aluminium ones are too weak and may bend in a heavy seaway...)
Just measure the length you need and the inner diameter of the holes you want to use.
Drilling the anchor shank isn't really a good idea, if you need to drill then drill through the bow fitting to marry up with a suitable existing drilling on the anchor shank..
Your idea of using the same hole as the anchor shackle along with the two on the end of each 'cheek' of the bow fitting as indicated by the red line is probably by far the best solution
..Bill.
