We used the floor from our donor 1965 Chevelle 4-door parts car to cut some patches to repair the floor on our '65 2-door Chevelle.    The damage was minor, small pinholes left from leaky windows, so we thought patching using butt welds would yield the cleanest and easiest repair.    Here's some shots of the process.

Once the body was on the Autotwrler, we had the underside sand blasted to clean the rust and reveal where we needed to cut patches.    Using a cutoff wheel to keep the heat down, Scott cut out the rusty pieces of the floor.   Then, we plasma-torched out the corresponding areas of our donor car and trimmed the pieces to fit the holes in the car.  

Next, Scott used a high-heat, quick tack method of MIG welding to butt-weld the pieces in.   This process creates large, flat tack welds that penetrate quickly and require minimal grinding.   Once a weld is made, Scott moves to a different area to minize the heat and warpage.    He also pauses for several seconds between each weld.   

Once the welding is complete, a quick pass with a grinder and 3M Rolloc abrasive discs cuts the high spots down, leaving a flush surface.  

We sprayed some weld-through primer on the finished piece to illustrate the fit.   Further cleanup and bodywork could be performed to make it perfect, but undercoating would probably hide any tiny grind marks left behind.   

Sources:

3M

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