The header extensions are supposed to increase power and torque. I have never checked with them on vs. off to be honest but thats what the guy who was a very serious street racer in South FL recommended after I destroyed the 3" Pypes full exhaust system along with a driveshaft on my first track outing ever. I also had a very memorable 180 rotation at about 1,000 feet thanks to the trans fluid but no damage. Lesson = dont use nearly 60 yr old  stock upper control arms. I'm a rookie at the racing stuff.
Here's some commentary.
https://www.yellowbullet.com/threads...sions.1182330/
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1965 GTO Cameo White, Red Interior - 389, ST300 numbers matching
1965 GTO Black, Black Interior "Post" car - 389, M-20 numbers matching
1965 GTO Silver, Black Interior 100% dedicated drag car w/1968 400(461), 16x Heads, TH400, 3.90 9" Rear 1.656 60' 7.566 90.057 (1/8th)
1971 GTO Black (orig. Quezal Gold), Gold Interior - 400, TH400 numbers matching
1972 GTO Cardinal Red, Black Interior - 455, TH400 almost numbers matching
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