Another thought I had which may eliminate the potential rust issue in overlapping metals, is to weld some flat bar to the bottom of the rails, perhaps leaving the ends open near the side panels and holes below the engine mounting nuts. It would be hard to paint/coat inside the rail nowt that it is effectively box section, but not impossible. Plus, it would tie the front and back sections of the rail together for a bit of extra rigidity. I'm no engineer though, so I'm sure there are drawbacks.
Cheers,
John
