Re: Chainplate fabrication
We took our shroud chainplates out one at a time so the machine shop could match the hole placements and the angles of the bends. In Mazatlan there is a shop with a water jet cutter that did a great job of cutting and bending for about $100 each. Someone else will need to advise you on what services are available where you are. After you get the bolts out you can just pull on the shoud and it will extract the chainplate from the teak rubbing strake.
We used a Fein tool to clean out the old sealant on the rubbing strake and to cut the teak slats on the interior for removal. If you cut right on the screw holes you can reinstall the slats without too much noticablle damage. The overhead panels need to come off so you can reach the nuts on the bolts. A big pry bar is needed to push the bolts out from the inside. You'll need several tubes of caulking. We used Life-Caulk by Boatlife. When re-installing turn the nuts rather than the bolts so the caulk in the holes stays put.
The old chainplates are 1/4" stainless but 5/16" will fit in the slots in the teak rubbing strake which gives you a stronger replacement than the original. You need to decide what bolts to use before having the holes put in the new plates. The originals had the square base of a carriage bolt in the round hole of a chainplate so the hole is oversize for a 1/2" bolt. We opted for 1/2" holes and 1/2" bolts so there is a good bearing surface. I agree with Niftynickers that 316 is the preferred material for the plates and bolts.
We had rust streaks after after a while because the top holes were not polished. Make sure the shop polishes all edges and the outside faces of the plates and the insides of at least the top holes.
lying Lake Union