67-76 floor into a 65...
I believe the crossmember is complete side to side and the center supports the floor. So aligning the floor cut and adding a extra support for the different year flooring should work
What Stephen is talking about is that fact that the floor completes the cross member as a boxed structure in the same way the frame rails and some frame connectors use the floor.
Think of it as a "torsion box" not to be confused with torsion bar. Torsion box designs create an extremely rigid structure much like how honeycomb composite panels are made and for the same reason. Light weight, easy to make and cheap. A cardboard box is much more rigid when the top is actually taped shut.
The problem I am trying to avoid by replacing (possibly) the centered hump, is limited transmission choice. Right now, the only thing that will fit (I think) is the 904 or a manual. No way in hell will an overdrive of any type fit. Since I'm replacing the floor, my thought process is, at this time, to "fix" the problem as I see it to "future proof" it for other options.
At the end of the day, I need to account for the narrower frame rail placement which affects how the transmission cross member is attached. If I cut equal amounts from the center point, I need to get even more creative with a cross member. It might simply not be worth is and the simple solution might be to simply add material from the existing hump, up to the floor and be done with it.
That's just one of many options I'm looking at. Another is to cut the hump out of a newer A body and graft that in, but I still have the width issue and trans cross member to deal with.
Thanks