Yup, that's the C&C smile. I had a 1974 C&C 27 for 20 years and I am familiar with it. If you could remove the nut by hand it was REALLY under torqued and I'm sure that contributed to the crack you are seeing but even with proper torque it will still happen. I torqued my keel bolts every spring after launch and still the smile came back. G-flex seems to be the popular repair material. Peel out as much of the previous repair material as you can, fill with G-flex. Since you are removing and replacing the nuts you will need to torque them on land but check them again after launch. I'm not sure if you should apply full torque on land or not.
Another issue that can contribute to loosening the keel bolts is the swept back keel C&C went with in the early boats. It looks fast but isn't really that fast. Unfortunately it can cause uneven loading on the bolts if you put too much load on the keel. You really should use a cradle that properly supports the hull and doesn't put the hull weight on the keel. The idea is to only have the keel support itself, which can be hard to do using jack stands. If not evenly loaded, the swept back keel can try to pry the keel away forward and put a lot of compression load on the aft end of the keel. The good news is the hulls were really overbuilt at the keel stump and can take a lot of abuse.