ST 300's were what came with the 2 speed 67 Firebirds. The transmisssion is actually a heavier duty than the powerglide. We have a 67 with the same setup, and it was an original product of the car. Because of a big fire at GM back then it was hard to track what actually went into the vehicles. But all the research we did shows that you have the correct tranny for that car.
