Keel Bolt inspection / replacement

[Kaptin-Jer]

I have a Beneteau Idylle 11.5 built in France. Has anyone had any experience, or knowledge of the method of Keel bolt fastening used? Were they cast in place in the keel, or tapped through the bilge? I have written Beneteau, but haven't received an answer yet.

The nuts (or heads) and washers are badly rusted and I want to pull one or two to inspect the bolts, but I don't know what method was used and I don't want to shear the bolt if it was cast in place or through bolted to the bottom of the keel. If they are simply tapped from above could you let me know the size of the bolt I should expect?
What method of repair have you had the most success with? Replacement or adding new bolts? If the answer is to add new bolts can you direct me to a web site, or book, that will instruct me in the correct method of determining the size of the bolt to use and best installation method?