GT’s website has the 54 stack at 555 and reach at 379. Regardless, unless you are pushing the envelop on being too big or too small for a 54, I don’t think it’s as critical as people want to make it out to be. The adjustability of cockpits and seat posts makes it possible to fit a couple different size bikes for most people.
Without knowing more about your dimensions and seeing you in person these things are a bit subjective. Sounds like a good deal on the S3. Great bike.