Dongray - I would recommend two options depending on how much work you want to do:
1. Cut the existing floor just forward of the seat riser flange. Cut new floor panel to butt up to the cut existing floor and spot/butt weld the seam.
2. Drill out spot welds securing the forward edge of the seat riser flange to the floor. This will allow you to separate the seat riser and floor enough to cut the existing floor from below without cutting into the seat riser flange. Spot/butt weld the new to existing floor panel seam under the seat riser. Plug weld seat riser flange to floor panel.
I hope this helps? Others may have different methods, but that's how I would do it. Take your time and space out your spot welds so as to not warp the panels. Good luck!!
