I'm trying to wire a kill switch but am stuck on how to get the wire through the firewall without drilling a new hole. A buddy of mine said to run it with the speedometer cable but that seems like an odd choice. Anyone have any other recommendations?

 

If you want to go real trick, customize power door locks, then have a servo on the same frequency, in the engine bay that operates the kill switch, that way when you use the fob to lock the doors, it will also disable the ignition.
Idea inspired by your user name.

 

find the wire from the ignition switch that goes to the starter relay, this is inside , cut the wire and put your switch in there. it wont kill the engine, but if your using it to prevent theft, it will prevent cranking the engine from inside the car. unfortunately, no matter what you do, with a few alligator jumper wires and some know how anybody can steal a vintage car without the keys

 

The best way is actually using aftermarket ignition systems. They typically have a way of wiring them to prevent the box from charging the coil. They will allow either a magnetic/optical pickup OR a points; when both are grounded the box won't send power to the coil. So you wire a switch to whichever you're NOT using, and when it's closed the engine will not run period.

As for where to run the wire, at some point you're going to just start drilling extra holes to wire things through (if you get to that much wiring). The speedo cable is not a bad idea though.