I've been down the keel bolt replacement road, and I have some concerns that I would address before drilling holes in the bottom of my boat.
The keel on my boat was made by I Broomfield & Sons. The keel bolts are set in - for lack of a better word - pedestals in the lead, with islands of foam between them... Here is a pic that should explain what I mean;
That brown stuff is polyisocyanurate... It's not really very strong stuff, especially with a tensile force applied. The white stuff is 5200, and you can see why the keel was dropped by looking at the bolt in the background.
I believe that simply drilling a hole without KNOWING what you are drilling into will be akin to this;