What this guy said....except, if I recall correctly, it is stated most everywhere that exhaust must exit behind the furthest most passenger, which is at least behind the cab on pickups. (If it were the rear bumper, then why do factory "off road" packages generally dump before the rear tire?) I would think the primary concern is inhalation of the "carcinogenic" fumes.

Seems like a hood stack would amost be force feeding fumes into the cab's ventilation system... To each his or her own, though...