I just finished this job. I have the 2 piece set-up as well. All I did was bold the chainplate to the bulkhead, then went topside and squished butyl tape into the chainplate slot through the deck, around the chainplate. I then made a bed of butyl tape under the area where the flat chainplate topside fitting comes down to the deck. I seated the flat plate over it and pressed down hard to seat it into the butyl tape. I then screwed down the flat plate onto the deck, as butyl tape squeezed out under the plate. I then rolled a thin line of butyl tape, and wrapped it around where the chainplate meets the flat plate, on the top of the plate. Looks a little like overkill, but seems very watertight so far.
Before all of this, I taped the underside of the chainplate slot, and injected 2 part penetrating epoxy, thinned with denatured alcohol, into the slit to let it soak into the core. After about 2 hours I removed the tape, and a small amount of remaining epoxy drained the slot. I had to push with steady pressure to run the chainplate through the slot, which had been narrowed by the setting epoxy. I'm happy with the end result.