Verle 790 keel bolts

I am refurbishing my verle 790 class yacht after I ran aground last year. The boat was built in 1979. I took the keel off and uncovered a can of worms. The bolts were twisted and the holes they came out of the hull were much bigger than the bolts. There was even an extra half hole at the forward end. The boat was written off by the insurance. I decided to replace the bolts which are made of cast iron. But I can’t get then out after heating them up. A blacksmith couldn’t get them out. No one can tell me if the bolts are screwed into the keel or are permanently fixed. So I’m drilling two out and planning to replace them with new bolts by tapping new threads into the keel. I’m curious doesn’t anyone know if the bolts are permanently fixed or do they screw out?

In view of your issues, would you suggest strengthening the keel to boat body, I’m looking at one of these but if the keel peels off so easily could you recommend this boat?

Cast iron bolts, would high tensile steel or stainless bolts be more practical?