adding a NSS

A older motorcycle brake light switch works great for a clutch switch.
It takes a little experimenting with the spring from your linkage to the switch, but it works well.
The switch makes contact when pulled by the spring allowing your relay to send power to the starter.
The spring is for when the switch gets to the end of it's travel it stretches out instead of ripping the switch loose from wherever it is mounted.

The goal here is that the car will only start with the clutch in. (you probably figured that out already though) :-)