I'm not being critical of you. I am simply stating that I
would not drill holes in the bottom of my boat unless I knew exactly
where to drill, and what I was getting into. I believe that the only way to find this information out, is to drop the keel. My decision is based on my experience.
However, we are discussing your boat boat, do what you will...
I believe that the foam goes down about 4" into the keel, but I don't know exactly, because the repair
work (new bolts) was done by I Broomfeild & Sons in Providence, RI.