Before lifting the cab all the way off the frame.
I would line-up the front floor cross members mounting holes with the frame holes.
Then weld some bracing along side the original cross members, from rocker panel to rocker panel.
I know what you're saying. Sounds like a great idea. It will be a while before the cab comes off. Sure would be a bummer if I took the cab off, put in a new floorboard only to find that the front mounting holes would not line up.
I still have to make a rolling dolly.