I am replacing the middle floor brace on my 70 Chevelle, the original has rotted out. First, what is the best way to replace the brace, cut out the floor and weld the new one in? Then weld a new floor piece back over? Then what should I use on the new floor brace to keep it from rotting out. Haven't ordered the brace yet so I'm not sure if it comes with any rust prevention.

No rust protection on new part. If your floor boards are in good shape drill spot welds out and replace. As far as rust prevention i would sand the inside of the brace with some 180 dry paper and epoxy it. Not spraying the welding edge and using zink cromate primer for the welding edge. Clean scrub Clean scrub sand sand sand sand sand the crap out of the finished project and primer with epoxy primer. A couple of coats like four of them. Let it dry then spray some semi gloss paint over your work, Krylon spray bomb will work just fine being that the epoxy is your protecting primer. Under coat works just fine over primer. Good luck just get into it.