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Don, I think silicon bronze is still considered the most corrosion-resistant (and possibly expensive) metal for the job. The I-28's were built quite heavily though, and unless there is a definite problem you may be on a snark hunt there.
IIRC one of the keel bolts is also blocked by the mast compression post, which transfers to a 4x4 which forms the aft inboard corner of the head compartment, which sits on a "bridge" that blocks access to a bolt, so you are going to have the "mandatory opportunity" to rebuild the head compartment while you are up there, and the flooring under it. (I think there are or were pictures of head/step ebuild of an I28 on the web someplace, don't know if they're still up.)
By the way, I've seen Robert Perry's original drawings for the I28. He designed it with a keel-stepped mast, not the oddball offset into the head that was actually built. The actual build apparently wasn't the same as the original design, in a number of ways. Perry is still corresponding and consulting on these boats--for a fee.
If the only degradation is to the threads that are clear ABOVE the nuts...I'm not sure a project of this magnitude is called for. Compared to a lot of other boats, Islander used a generous number of keel bolts to start with.
And remember, drilling into lead will raise and distribute lead DUST, you probably should be doing all this work with a suitable respirator and ventilation, and then doing a hazmat clean-down in the boat. Lead dust is not a good thing to be breathing, or wearing.