5.2 magnum swap--which gasket do I need?

Yes, the 273-340 pan will fit (people say, I have never done so), as will the associated gasket.

No, the 5.9L gasket will not fit. It fits only the 360 (and 5.9L) pan, which is different.

You didn't ask, but we must infer that a 5.2/5.9L pan will fit a 360 LA block.