I'll guess your rusted out spot is on the top of the rail just as it kicks up to go over the rear axle. Repairable, but read on.

If you're missing the X member then you might as well look for another frame. They're available all the time in the neighborhood of $1-3000 dollars, rolling, depending on condition. Ebay often has them listed as an example. Also check with your local chapter of the Early Ford V8 Club. Visit a free classified site called www.fordbarn.com ,new listings there all the time, and you can post a wanted ad for yourself.

For a convertible you need extra rigidity. On my '36 roadster we added some boxing plates on the side rails, and on the X member legs (with openings to facilitate exhaust routing), and then welded in box section (1x2) tubing to replace the riveted supports that are near mid frame. Personally I would go with the 9" just because it's simpler and less costly in long term maint. than the Jag. Figure out who's air bag system you prefer and modify the frame according to their instructions. I would strongly suggest Heidt's........hard to beat for quality.