Nicknamed the Shotgun, the Boss 429 engine was so big that Ford contracted its performance engineering vendor, Kar-Kraft, to modify the Mustang’s shock towers, move the battery to the trunk, and make the big-block fit.
Read the full article at Hagerty.com: https://www.hagerty.com/media/magazine-features/25-greatest-mustangs-1969-boss-429-mustang
Disagree with the 375 H/P just like I disagree with 425 H/P designation for the mighty 427 side oiler. The sole purpose of these low advertised numbers were to try and fool insurance companies. These companies used to send experienced drivers out to dealers dressed as customers to drive these monsters, they were not fooled.