Re: Replacing lead Keel bolts
on a 30+ year old boat, I certainly would not use some threaded rod and a few nuts....were it an iron keel you may have gotten away with "unscrewing" them and replacing with the correct pitch bolts
IMHO this is a wake up call to have the keel repaired or replaced by a reputable mechanic or yard with experience. The threaded rod generally available today is junk, not coated or strong enough for your task. The original bolts were likely "L" or "J" shaped and quite stout stainless or coated steel and cast in place as the keel was poured. If you do drill holes be aware that hitting that hidden stainless will likely walk the bit around and out where you really don't want it to...that assumes you can find a decent bit long enough.
Same with any blind insert that will be inserted from the top, If two were bad, you can be assured the others are not too far behind.
For your safety, get it fixed correctly