Yes, good idea to replace with stainless backing plates - note that I say plates and NOT washers since the washers do not distribute the load as much as a large stainless backing plate - just make sure there are no sharp edges or corners on the backing plate.
i have yet to find keel nut torque specs but - i suspet around 100-150 ft lbs would do the trick - also place two stainless nuts on the bold and use loctite.
Since the list has had no activity in quite a while, I thought I'd start a new thread. For those of you S2 owners who have boats with keel bolts, how many of you now have rusty mild steel washers on your keel bolts? Anyone have an inkling why S2 used mild steel washers instead of stainless washers? The gentleman who surveyed my boat indicated that I should swap the mild steel washers for stainless. Wadda y'all think? Good idea, or...?