Welcome to FABO... :welcome:
I am no body man, but I would think that you want to get the solid frame rails in first, then the new floorboards...
You may need to remove the floor boards first to be able to access the frame rail sections that you need to replace... Sometimes there are spot welds on the frame to hold the floor board... You have to break those free so you can remove the frame section that you are working on..
Also keep in mind how you are going to support the car while replacing the frame, you don't want it to sag before getting the new frame sections in and then the body geometry will be off...
