storing my car, what should I do about the rust?

[Xez]

That looks like a classic bit of rust - the same happens to Capris, but not quite that quickly Screen out, rust cut out, plate in, filler and paint.

http://www.capripower.co.uk/forum/index.php?/topic/5779-3-0s-resto-for-2010/page__st__180

as an example.

What I would say also. It may not require new metal if you take the screen out and grind all of the rust away.