Those one-piece rails are going to be your most difficult fab work. I wonder if they're worth it. The weak point in your three-piece lower plate is obviously at the joints (and what you said above about sandwiching all four layers at that point with M4 screws sounded semi-convincing to me
). And it's at that weak point that your one-piece rails have their kinks. I'm not sure that turning your previous three-piece rails back into one-piece rails is going to add any rigidity to the chassis. I imagine the kinks in the rails would allow the rails to bend up and down at that point (where straight rails would not). So, I don't know if it's worth all the effort to make them.
Those are my thoughts trying to imagine how it would work when stressed - I don't think the one-piece rails would be a great advantage over the three-piece rails in reducing the stress on the CF top plate.
My vote: try your previous plan (3-piece bottom plate and rails) and if it fails go for a one-piece bottom plate.
So - when are you expecting to get your truck? (or is this just an elaborate thought-experiment?
)