On my RV-3B, I used Howe Fittings, the wooden gadgets, to help with the shape.
These changed the angle of the tiedowns and gave a better shape.
If the leading edge portion of the rib starts to crush down, you can use supports like these. These were on the outboard part of the wing, but the same would work for the tanks.
The threaded rod kept the ribs in position.
Dave