I just replaced my keel stub and installed the lag bolts a little over a month ago. I also have a 76. Hull # 249. I found more layers in the keel stub than Catalina described and showed on the drawing they provided.
There were actually two layers of plywood separated by REALLY hard grey material. It's harder than concrete although it looks a lot like it. I tried a ton of different tools to remove the grey stuff. Nothing worked well. I ended up having to beat it out with a hammer and wood chisel...took forever. There was another layer of the grey material below the second layer of plywood. Then finally fiberglass and lead below that. In all, I ended up cutting out about 4 to five inches of keel stub.
FYI - The lag bolts were extremely difficult to install in the 27/64" hole. My pneumatic impact gun couldn't drive them down. I had to bore out the holes a tiny bit. Still couldn't get them in all the way. I finally borrowed a massive Dewalt impact gun and it even struggled with one of the lag bolts.
It's not a fun project but I'm really glad I did it after seeing all the rotten wood that came out of there.
My complete refit is taking completely too long!