It was originally a V-8 car. The number after the "U" is a "1." That's a V-8 car. If it was a "6" after the "U", then the car would have been an OHC-6 car.
In 1967, the TH400 only went on 400 cars, otherwise the ST-300 (which is what you really have, not a Chevy powerglide) went with the 326.
