As a non-engineer I can't really make a suggestion about sizing, but if you are replacing them anyway I would highly recommend switching to titanium chainplates. I sell titanium chainplates and hardware, so it is defiantly a self-interest thing, but they can be stronger, smaller, lighter, and immune to corrosion for a relatively small premium.
Generally we recommend either having an engineer spec alloy and size, or replacing size for size with Grade 5. Grade 5 is massively stronger than 316, with roughly 5 times the yield strength, and about twice the tensile strength.
Some people have also used grade 2, which has Slightly higher yeild strength, but only 5/8 the tensile strength. They do this by going up in size to compensate for the lower tensile strength. Typically it doesn't take a massive increase, and not much more in cost, since the machining cost is the driver or purchase price, not the quantity of material (within reason of course).
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