Wow!
Beautiful truck!
Welcome to the site, you've found the best place for lovers of stock Chevrolets.
I've seen light wires etc short or ground to the point of burning the wire off before, but never seen it kill the engine in the process.
You've got something really unusual going on.
I'd start by tracing the wiring supplying the ignition circuit all the way from the starter post to the coil, and look for things peripherally connected that shouldn't be there, or things the circuit is running or bypassed through that it shouldn't be.
Good luck 🙂
PS
How does the ammeter react when you apply the brake and this happens?
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