To All -- I'm very thankful for your replies. Since this will be a one-time choice, I want to make the best choice... best rigidity, best ground clearance (favored same as current unibody frame rails, or not much lower), attachment methods (welding is allowed, but only at front and rear, along with another point in between... such as the Magnum Force does to the floor); floor boards cannot be cut or modified (perhaps some non-noticable dinging could be gotten away with); weight is also a concern, but maybe they're all about the same?...(I'm willing to add the weight since its a low C/G and centered withion the vehicle).
I'm planning to do this perhaps in 2015-16, so, I have time. Going to the car shows is one good place to see what other E-bodies have for SFCs.