You don't really mov the floor up, you move the body down. Hence being called a bodydrop. The floor stays in the same place in relation to the frame. The body drops around it, then you add the fillers and make it strong again.
Well now what in the world did you do there? Looks like you made room for JUST the frame rails to tuck up into? Could you explain what it is you did there?
Its called channelling. You lower the body mounts on the frame, and cut the cab where the frame would hit. Then fill that area in. Totally different approach.