Hi mightygal,
You may get various opinions from other owners, but I doubt the factory will give advice on modifications like this.
If you install it in between the transfer switch and the main circuit breaker panel, it will protect against faults originating from shore power, generator, the main power cord, and a failure of the transfer switch itself. Also, you'll only need a little #6 wire to make the connections and won't have to cut the existing wiring.