Sorry I don’t have a NC42 but an NC43 which is a different boat. What I can do is share my experience on my NC43.
I spoke to Veli (email@example.com) at Nauticat in Finland who sent me the plans for my boat which shows where the keel bolts are. Mine has about 18, which were all hidden: some under the false floor under the engine and prop shaft, and the others under the water tanks.
I had to remove the engine and water tanks and then cut away the floor all along the top of the keel. The problematic ones were right at the rear of the keel (I ended up cutting sideways into the keel above the nut to gain access), and one under the mast foot which is composed of iron punchings in resin and was seriously hard work to chisel away.
I undid all the nuts, suspended the boat in the lift and used a wiresaw to cut the lead keel away from the stub. Once on the ground, cleaned it up, and used Saba primer and sealant to refit (you need a lots of chaps to help with this to prevent the sealant curing before you can get it all back together…)
It was a worthwhile project as many of mine were leaking and some were loose (not even hand tight…) however my boat is 1984 so I suspect much older than yours….
Apologies if none of this is relevant, but hopefully helps.