Why are you doing this?
If there is something wrong with the existing chainplates, whatever it is may well affect the hull integrity. Were I a rigger in your boatyard, I'd not answer the question about how until I knew the why.
If you really must go with new chainplates you should go external. The chainplates need to be through-bolted to a nice long ss strip anyway, so there will be something outside the hull. That might as well be the chainplate itself, and save yourself cutting a hole in the deck.
Do you have access on the inside?
I don't know your boat, but there's plenty of production boats where the chainplate comes in through an athwartships slot and is bolted and/or buried in a bulkhead rather than the hull. However done, there may be a ceiling or bit of cabin furniture in your way. Sometimes it's impossible to get a continuous ss strip for the bolts to land in. It can be a bit discontinuous but the more unitary it it, the better load sharing you'll have.
You of course need to know if the hull is cored at that point. If it is, this instantly becomes a more complex job as you'll need to over-bore the bolt holes and makse some non-crushable bushing, which could be as simple as a bit of structural wood.
Anyway, there may be more here than your question reveals.
G'luck
Ian
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