Place it right after the exaggerated claim, so the doubt answers the boast directly. Follow it with Oh My God Wow when the story gets even wilder, or use No No Wait Wait just before it while the speaker tries to backtrack.
About this sound
This is a short spoken line addressed to Tristan, delivered in a flat, doubtful tone that makes the disbelief land instantly. It feels sceptical and playfully dismissive, like catching someone mid-exaggeration and refusing to go along with it. The clipped delivery gives it a deadpan punch, so the doubt reads clearly even without any context.