Guys, if a quality 300 series stainless steel bolt is used in the keel, there is no way it will shear. A 1/2 8.8 or 10.9 316 stainless bolt is silly strong, you will rip out fiberglass long before you break one of these bolts. (you can google the shear strength and see)
However, if you neglect to check your bolts and they wear half way through, yes you could break a bolt - but that's not the bolt's fault!
Joe, by pressing a bronze bushing into the bore of your keel you didn't really reinforce it, since the bronze is softer than the steel. What you did do is provide a wear surface for the bolt as the keel pivots. This will wear the bushing and not the bolt, so you will need to asses your bushing periodically (once a year or so?).
Is a bigger bolt necessary? No, since the keel pocket will fail before a factory sized bolt will(1/2 inch). Now, a bigger diameter bolt will give you the advantage of letting it wear through some before it reaches a failing point. I suppose if you don't ever want to check you keel bolt you could put in a really big one - but it's best to check on it periodically.
Flat.