[goatwgn]

Originally Posted by geeteeohguy

Just swapping in a TH350 for the 2 speed ST300 will be a night and day difference. The car will literally feel like it has 200 more HP. Mainly due to the lousy 1.76 ratio of 1st gear on the ST300. The TH350, being a 3 speed automatic, has about a 2.75 first gear. Regardless, an ST300, while super durable, and original, is a serious performance retarder. It's like running the 440 with your shoes tied together.

X2 on this. TH350 (2.52/1.52/1.0), TH400 2.48/1.48/1.0), both have the "first gear" that the ST300 (1.76/1.0) is "missing". Best modification you can make to the car. There is a reason a lot of GTO buyers bought the manual tranny before 1967. And if you are up to the manual swap, all of them are better geared than the ST300, and are way more fun to drive.