Mercury Cougar... darn close to a Mustang in a couple ways that aren't obvious

the Mustang was project T-5, the Cougar was T-7

The SCCA Championship (over 2 liters) was won by the Mustang, but the Cougar team was only 2 points behind in the 1967 points run https://en.wikipedia.org/wiki/1967_Trans-Am_season