While most of the Mustangs in GTS are factory built Boss 302's several are not including the one that won Detroit last year. Ford will sell you all the parts to build a clone Boss 302 including a BIW chassis with the cage in it. In my case I bought a 2011 Mustang V6 raced it for 2 seasons in a club class then converted to a Boss 302 that I still club race, but could enter a WC GTS race if I wanted to with the addition of the wing and front splitter. In the end my build was cheaper than going the factory built route since we did the work in my shop.
Cheyne