Ladder style frames are weak by design. Heavier material helps but will never overcome the design. Triangulation, stressed skin, cages, etc all help a bad design. Some are better then others. Not one of us will ever know for sure which one is best
Frames should not flex! But I am sure there is a point of diminishing return. I would think that some of the replica frames rival or best the originals, some don't. I would think that the new cars from Shelby have stiffer frames than the originals. Point is, you can make some good guesses looking at a frame but that is all they are, guesses.
Rick