Unlike 240Z replacement floors, the 280Z replacement floors I've seen are almost nothing like the originals. If you looks at the subframe connectors bad dog sells, the 240Z rail is flat, and has a flange, the 280Z rail has no flange, and is contoured to mate to the floor. There's a handful of bead rolls that do go over the frame rails, as well as a couple steps, curves, what have you.
As for stainless, I've seen lots of people suggest stainless for various things, and based on what I know about stainless (not a whole lot), I'd say stainless is sometimes used inappropriately, ie. stainless bolt kits. Stainless isn't just steel that doesn't rust. It has different properties that make it inferior to steel in certain applications.
I agree with seattlejester, leaving the ends open isn't a good idea. They're basically just dead weight if they're not connected to anything, and totally overkill for stiffening the floor. Honestly, I didn't even know you could weld stainless to mild, I thought you had to braze it.