I don’t think there is any issue with curving the floor. I don’t think there is even a limit to how much it curves as long as it meets the minimum floor thickness.
In the fuselage interpretation dated 10-17-83 ammended 1-19-2013 with respect to the hullside profile: “the term proportional will be defined as a smooth curve without reversing the line abruptly”. While not specifically referring to the bottom edge of the profile, I think it should apply to both the top and bottom edges.
I don’t recall seeing any construction in recent history where the side panel was slotted or notched for the plank. At one time it was common to locate the plank without fasteners by adding blocks to the bottom of the hull, but never a notch in the side panel that I can recall.
If we allow a notch in the bottom edge of the profile at the plank, I think it will open the door for notching the bottom in any location. I do not think we should allow this.