I did this repair to my Paceship PY26 about ten years ago. Was a long process, but to this date I have no rust or cracking of the keel joint. Now I do sail on Lake Michigan, so no salt. But I think it is encapsulated enough that even in salt water I don't think I would have a problem.
First I focused on the keel Joint. I used a flexible sanding disk on my 7 1/4" grinder to sand down and dish out the area around the entire keel joint, making sure I got down to bare metal on the keel. Next I took the 7 1/4" grinder with a 1/4" thick grinding disk and plunged in about a 1/2" all the way around the joint. I then re-torqued all the keel bolts and filled the gap with 3M 5200. I let this dry again for a little over a week, as the instructions say it takes 7 days to fully cure.
Next I ground everything off the keel down to bare metal and sanded off any excess 5200 around the keel joint. I then applied an off the shelf spray on rust converter. I did several coats and let it dry for over a week. I then applied two coats of West System resin as a barrier coat. I used the hot coat method to apply the second coat, so that I did not have to worry about amen blush forming between coats.
Once the resin was cured I focused back on the keel Joint. I applied several layers of glass mat and cloth in various widths to fill in the dished out area and reinforce the joint. You want to start with the widest piece first and work your way up to the narrowest.
Once the keel joint was reinforced and had a chance to fully cure, I gave everything a light sanding and began the final fairing process using West System 410 Ultra Light fairing compound. After final fairing, I applied five coats of Inerlux Interprotect 2000 barrier coat and finally the bottom paint.
Here is a little tip on applying the fairing compound to save on material and sanding. Apply your first few coats of fairing compound with a notched trowel. You can pick up cheap plastic ones at your nearest hardware store. This way when you are doing your initial shaping, you are only having to sand the ridges off. Again, in between coats, make sure you are careful to remove all the amen blush prior to applying another coat.
Here are a few pictures of the process. Hope this helps, and good luck