I had a 1978, had the same type of cracks, turned out to be the base of the wooden compression post had rotted away. The fix was to loosen the standing rigging, put some wooden plates beneath the cabin roof around the top of the compression post, jacked it up about a quarter of an inch, then cut away the rotted base of the compression post, replaced that segment with aluminum blocks and carefully lowered the post back in place. Turns out that this was not an unusual problem. The compression post if not a solid post. Instead, it consists of four, 1-inch thick, 3.5-inch wide boards glued and screwed together. The post is then anchored to the bulkhead wall of the head. When the bottom rots out it places a lot of pressure on the bulkhead and floor area outside and adjacent to the head.
My keel nuts and bolts were stainless, and while they were somewhat discolored they never rusted. Those bolts might just be real cruddy from being constantly submerged.