The th400/np203 combo was available in mid/late 70’s square body 3/4 or 1 tons. This 32 spline 203 is widely used in Doubler applications.
there werent many tho
GM Clutched their pearls on using the full time only with a Turbo hydramatic BUT they also clutched their pearls on retaining NP205 for manuals. Maybe it just depended on which pearl and when was more important to clutch lol
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Not much can get you an adapter for a TH350/np203 to make it th400 and thats not including the shaft for the np203 but i imagine if someone is doing this work a shaft change would be not very unreasonable since they may be rebuilding anyway.