10-24 (3/16) bronze usually come in 2, 4 and 6 inch. If you need longer, they may be out there somewhere, but usually you have to jump up to 1/4 inch. As for the hole, if it were me, I would plug fill the hole then re-drill through that so you have new wood for the fin bolt bite into, then bung the upper portion over the head.
Brian – You asked the question do I really need to take them out?” From the way the thread was going it seem like for some reason you needed to take them out, but now that you ask, for what reason would they need to come out? I cannot imagine why as long as you can get the nuts off and the seats and sports out. If you can do so, then of course leave the bolts in place, protecting the threads as Howie suggested. No sense in doing more work than is needed, and even more important, there’s no sense in possibly damaging the rails if you don’t have to.