Edited to say --ah--J-bolts! that's why you can't remove. Bummer.
Leaving all the info below--though it doesn't apply to you
Info might be useful to you: Our boat is a different situation than yours, even so, we had keelbolts through wood floors, keelson, and lead keel, as well as holding together parts
of the deadwood near the prop aperture. Ours were very old--76 years old at the time of removal
. They were made of Tobin Bronze (higher zinc content than Silicon Bronze, really closer to a brass) and some of of mild steel. The bronze ones were true bolts whereas the steel ones were drifts (friction fit). They were all in rough shape and took quite a bit of effort to remove. Many of the mild steel ones were heavily corroded at the wood joints but full diameter along most of the bolt or drift. Same with the bronze ones.
We replaced all of them with Silicon Bronze but really we could have used Tobin Bronze, Manganese Bronze (what your prop may be) as well. Our bolts ranged in original diameter from 1/2" to 1" and length from 16" to over 4 feet. The hull
is 54' long, wood forekeel, keelson, lead keel, wood countertimber all were bolted/drifted as needed so there were about 100 total and removing them was a pain.
Hole size should be close fit to bolt size. It is common to wrap a thread of oakum (or even more common COTTON) caulking around the bolt at the nut to act as a seal. This is successful if the hole size is close fit to bolt size. Otherwise, all sorts of tricks are used to seal up around the bolt--anything from hightech grouts to just heating
up the bolt and pouring a melted wax down to fill the gap. Clearly the grouts, epoxies, and modern goos have the common problem of making things harder to remove "some day" when you have to do it again.
I hope your "someday" will be far into the future. Many people do use various SS with some success. There are some corrosion issues for use of SS in salt water
and you'll just have to figure out if you want to deal with that. The only reason to do so would be cost savings as it is typically cheaper to use SS than bronze.
You're not in the USA, but if for CF members who are, you can get silicon bronze keelbolts made-to-length and threaded to your spec and very affordable (for silicon bronze) by TN Fasteners, one of our sellers, in the Schooner Chandlery marketplace.
Link to the TN Fasteners here
. We made and threaded our own keelbolts in 2007 and would have loved it if TN had had custom keelbolts at the time!
Best of luck in your project