[PaulK]

With the boat afloat, what will you do if, when you start drilling down into the bilge for the new bolt holes, water starts coming in? The keel/hull joint may not be watertight along its whole length. Even if the pumps can keep up with it, getting the new holes wet will not be helpful.
For threading, why not match the originals? They’ve held up for 50 years. Gooping them up with epoxy is probably a good idea, as would be putting some sealant in at the keel/hull joint to make sure the water stays out.