The cable ends have to be modified so you don't have to cut the firewall.
Here is a picture of the numeric racing cables.
It is not a close up as I like, but if you look at the place where the nuts are on the cables, you will see they are captured by a T shaped flange on the end of the cable casing. This is how you attach the cable to the shifter or the transmission.
If you look at this picture of my shift cables installed in my car....
Click to view attachmentYou will see there are no flanges. They have been machined off, and replaced with a second nut. That is so the cables would fit in the chassis holes for the shift rod and speedometer cable. The only hole I put in to install the shifter was the one in the top of the tunnel.
And the wooden block is to raise the center box up enough for the cables to pass under it.
Hope that helps.