316L stainless is readily available and the most corrosion resistant reasonably priced stainless. Plain 316 is not as corrosion proof as 316L but still pretty good. 304 is stronger but more prone to oxidation. 316L will last for many decades and would be my choice if were going to do it again. I went with plain 316 but I didn't to ask for 316L and they have been fine for the short time I've had them. You can order SS in 1/4" and 3/8" thick material in 1/2" increment widths. The supplier may even cut it to the needed lengths so you only have to polish the outside and drill the holes.

I looked all over for silicon bronze but could only find it in round stock. No one seems to carry it in flat bars and I talked with more than 10 suppliers throughout the country. You can get 954 aluminum bronze in flat stock but it's about twice the cost of 316L stainless. 954 bronze seems to be plenty strong, readily available and corrosion resistant though I couldn't find out a lot of information on it.

The rust stains on your hull probably aren't from the chainplates but from your rigging wire, btw. Still, crevice corrosion is a problem on external chainplates. Had to replace them on my last boat after 10 years in the tropics. The inboard chainplates on my current boat were still good after 40 years.