Umm.. dude! You're worried about THAT? (I looked at the pic first).
Is that all the rust the car has or am I missing something?
Paint it with EXTEND rust converter! It's fine.
If there's some on the under side, scrape it clean, paint the rust with EXTEND, and when it's all dry spray it with undercoating. It'll be good for years.

You should see the patch in my 87's floor. It's almost as big as the patch the '86 is about to get.....
When getting patches made to rusty floors, always have the repair piece welded in. Rivets are fine for holding the piece in place, but it has no structural strength unless it's welded. And use 3M seamsealer. It's the best and there's no reason to use anything else. Does stink like crazy though.
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Rob Bareiss in Rust Central, New London, CT