I pulled the carpet and found the expected rusty floor in my 64 Cutlass. The areas where it rusted out follows the middle underfloor brace, which is also rusted out. There are also man made holes in the upper passenger toe board (aftermarket A/C lines) and the tunnel has been hacked up. My parts car actually looks pretty good in these areas and I was thinking of using it for the floor repairs. Do I try to leave the floor brace attached to the floor when I transplant it or do it in steps (brace then floor or floor then brace?)

 

Separate the floor from the brace. Then you can see what is actually needed.
Instead of using a spot weld cutter use a grinding wheel (not a thin cutting wheel) to grind down the spot weld to the brace and peel the skin off.

 

My '65 has this problem also, I don't know where to start!