I am beginning to replace the full left and right floor pans on my mist green 70 SS. The floors are pretty bad as it came from Indiana. Also the middle floor brace is pretty tore up. I would like to know if I can replace the floor brace with the body on the car and can I weld the floor pan on top of the rocker panel or should I break the seam and put it back between the two layers of rocker panel? Thanks.
:confused:

You should split the rocker and do it like the factory did - drill through and use plug welds. It will turn out better. I replaced the back section of the rocker because where the supports welder to was also rusted.

I think you probably can replace the support with the body on but you may have to raise it a little with a jack. Shouldn't be hard if you can get the mounting bolts out - probably should replace the bushings anyway.

Install it JUST HOW the factory did it.

To properly replace those braces you need to lift the body off the frame. Loosen all bolts etc jack up body and slid a 4X4 across the frame one, up near the cowl the other back by the wheelwell.

You dont need to remove the body off from the frame just raise to gain access to the brace that is sandwiched between the frame rail.