I would probably just call a local transmission shop. TH400s should be on the menu everywhere. Call two or three, tell them you want a performance build and go with who you like best.
FWIW I also swapped in a th400 into my first gen, never bothered with the kickdown switch. It really just never comes into play. If I ever need to I just manually shift to second and hit the gas. More fun that way anyway. Even then, I think Ive needed to do that maybe 10 times ever. I think the kickdown comes into play more if you have an OD transmission.
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1967 Firebird 462 580hp/590ftlbs
1962 Pontiac Catalina Safari Swapped in Turd of an Olds 455
Owner/Creator Catfish Motorsports
https://www.youtube.com/@CatfishMotorsports
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