Ignition Cut Off Switch?

That WILL work, but it has some problems...

If the thief (or someone who's allowed to use the truck, but forgets/isn't aware of the kill switch) leaves the key in RUN too long (potentially, just a few minutes) with the switch in the KILL position, it could kill the battery &/or overheat the coil.  Why?  Because you're keeping the coil ON, making it essentially a heating coil.  It's only designed for high-frequency intermittent operation (very short duty cycle, below 50%).  Yes, it can tolerate being held on for several seconds without damage.  But as it gets older, that time gets shorter.  And even if it doesn't burn out the coil, it's draining the battery about like a headlight bulb would.

A less-risky circuit mod would be to open the positive side of the coil - keeping it OFF instead of ON.  Or open almost any of the ignition module circuits (so it never wakes up, or realizes the engine is turning).

This diagram shows a few ways to interrupt the starter with a BIG (high-current) switch:

Thanks for the explanation, Steve.

The issue is that I don't want to open the hood every time I use the killswitch. From what I've read online, the larger killswitches need to be close to the battery. Is that correct?

Your low-current kill switch can be wired from the engine bay through the firewall to a convenient/hidden spot on the dash; regardless of WHICH of those low-current circuits it taps, or how it accomplishes the kill. It's relatively easy to fish a few small new wires through the factory firewall grommets by piercing the harness wrap on one side, pushing a smooth probe (Phillips screwdriver, knitting needle, etc.) through the bundle of wires to open a space in the harness, and then quickly shoving the new wire(s) through that space before the original wires shift back into place.

The high-current switch could be mounted in/through the firewall (wires outside; key inside); e.g., in the passenger footwell.  Yes, for any hi-current circuit, the length of wire & number of connections should be minimized to reduce resistance/voltage-drop through the circuit.  So keeping the switch physically close to where it's spliced is desirable.  But with proper circuit design (possibly new larger wire), good connections (solder, etc.), and proper maintenance (occasional cleaning & electrical grease), even the starter circuit (the highest-current stock circuit) can be lengthened.