I made this modification in the same way that kid442 did. I was lucky enough to find an original ’63 Cutlass with the entire 4-speed setup still intact. I got the bellhousing, clutch, flywheel, pedals and clutch release mechanism, transmission and the partial floor pan/transmission tunnel. The factory did cut out the auto. trans. tunnel on the assembly line and welded in a partial stamping to accommodate the trans. shifter. You’ll probably have a hard time finding an original car that was a 4-speed though.