Hi Ben, I had Port Towsend Foundry, Pete Langly, cast Manganese Bronze midship, lower bobstay, and boomkin chainplates, for my BCC 072 .
It sounds to me that you would like to use ss to replace your existing ss chainplates. The advantage of ss over cast bronze from Pete, will be weight savings.
Pete’s castings are larger than the ss ones, thicker, and heavier, and of course will need to be drilled, which needs careful attention to drill them, from the inside, while clamped in place.
If I had to do it again, for the midship chainplates, I would use aluminum nickel bronze plate, and bend them to copy your existing ss plates.
I would perfer the P T Foundry to cast the, exterior mounted lower bobstay chainplate in Aluminum - Nickel bronze, instead of Manganese Bronze .
It has been long known that P T Foundry, is slow to deliver on orders, which causes some owners problems.
I don’t know, but I suspect that Talesin midship chainplates, that were cast in aluminum nickel bronze, would not quite be the same curvature as your Morse BCC ones .
The boomkin chainplates from Pete are fine, as is, but are not drilled either.
If I were to want to replace my midship chainplates with ss, I would not choose 304L or 316L , but instead chose a ss alloy more rich in chrome and nickel, like the alloy which is used in ss rod rigging, or Aquamet (sp-?) 22 prop shaft alloy.
Most of my sources say, reguardless of which ever ss alloy you choose, polish them to mirror finish, inside, outside, and on the edges too.
Don’t forget to champhor all the holes in the f/g and ss too.
Good luck, do it right once, and you won’t have to re-do it again.
Douglas, BCC Calliste