If the before number is 437 and the after number is 546, thats about right for 6psi. Maybe a touch low. A true stock 5.7 number is closer to 340rwhp though so yeah the dyno is way off but the delta between A/B is about right.
The true number is whatever the dyno says it is. To be honest, who gives a Bleep what the piece of paper says. Take it to the track. Whether the paper says 350 or 850, it wont affect the timeslip at the track.