Personally, This is what I would do!
I would sandblast the top of the iron keel or whole thing depending on size. paint
it with the best marine
grade primer for metal I could find, locally.
And for the hull
a good sanding
with an 80 grit just to prep the whole surface.
Tape off the edges before starting. Using 5200 get a 100% coverage eliminating any air bubbles that you can and mount the two together with the nuts not fully tight, align the keel straight and let cure for a couple days or a week depending on the size.
Then tighten up the nuts the rest of the way.
What boat is this?
it's about .25 - .75 of an inch around the flange
It sounds like a 20 -27 foot'r. There could be other things to consider.