I repaired/replaced the mast step on my '72.
It's not a big job, just a pain in the ass.
To the best of my recollection:
Under the wooden block you will find to plywood supports cut to fit sit in the bilge. They will probably be rotting from soaking up bilge water all these years. It's deep bilge under the mast.
I removed the rotting wood and fashioned cardboard models. Took the models to a wood shop and had him cut new ones from marine grade plywood. The shop had to glue 2 or 3 layers of plywood to make the thickness work.
I tested them to make sure everything fit properly then coated the marine plywood with epoxy to seal it. I then glassed them into the bilge. Done.
Under the wooden block the mast sits on you might find a stainless steel plate...I think the previous owner installed this on my boat, I don't think it was factory, but I could be wrong. If you find the plate, let me know, I always wondered about that.
I also took this opportunity to install a simple bilge pump under the mast way down deep in that bilge. Water collect there and it's hard to get out. I think my boat smells better for this
All in all, about two long days of labor, not including fabricating the new wood.
If you do it right, you will never have to mess with it again. Sadly, it's one of the few things the factory did wrong on that boat. Otherwise, it's a great boat!
Any questions, please don't hesitate to ask
I'm in Mystic CT.