For a street-driven TH400, I don't believe you can improve on the factory switch-pitch converter that was used in the 60s. It gives you the best of both worlds - high stall when you want it, and tighter hook up when you don't need all the extra revs, and of course much less heat going into the transmission oil.
Now perhaps one of our resident transmission experts (which I'm not) can tell you what other parts you need to swap from a switch-pitch ST400 so the converter will fit/work.
Ray
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