[Daves55Sedan]

Sandblast the rust out of the top side of the brace, then vacuum out and wipe down with cleaning solvent. Paint top side with POR or other rust preventative coating. Punch holes in the flanges (that mate with the floor panel) for mig-welding the flanges back to the new floor pan.
I would also paint the bottom of the new floor pan entirely, then fit it temporarily in place to determine final cuts needed to the floor pan. Also mark with felt-tip pin each hole drilled out thru the support flanges.
Remove the floor pan and make final cuts. Punch out or drill out mating holes you marked for the floor support channel flanges.
Final fit: Place the floor pan in the car and using hex-head sheet metal screws, screw the floor pan down to the support channel flanges. Go underneath and spot weld the edge of the channel flanges to the underside of the new floor pan.
Now go up and remove all the sheet metal screws and fill the holes in with the mig-welder. This will weld the channel to the floor if your welding skills are good. If you are new to mig-welding, go back underneath and fill in the holes in the channel flanges with weld. This will definitely attach the floor pan to the flanges.
Finish off by welding the new floor pan edges to the remaining existing floor pans. Clean all affected areas and apply a spray coat of primer to all bare metal surfaces.
It is now ready for overcoat once all other floor panels have been repaired as needed.