To be clear, we're talking about a bare frame sitting on three jack stands with no weight considered other than the mass of the frame itself, as shown in the Wescott Instructions, and you're opinion is that the frame is going to flex at C notch because the supports are on the back spreader bar? I think you'd better go back and review basic statics. If the C notch allows flex under that loading it would be sagging when you fill the gas tank!!