Easiest way is to measure back 210mm from the back of the original gearstick hole & cut the tunnel down to the crease on the drivers side.
Cut along horizontal & then blend back up into the original gearstick hole - do this on both sides.
Then you use a scissor jack to jack the sides of the tranny tunnel out to suit the CA18 box & weld up the hole using sheet metal when you're finished.
Beats cutting the whole transmission tunnel out of another car!