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I installed it on my 74. Made very little difference. But again, I set it for "street" setting. If you are going to install it, drill the hole out and set for "strip". For $30, you can't go wrong.
Make sure you have heavy grease or vaseline when you are putting the tranny back together. I think there's 6 ***** in there you have to keep track of...
Do not, let me repeat, DO NOT ever use vaseline when assembling an automatic transmission. It is quite possible it will clog either some of the passageways or the filter or both. A product called transjel is the only thing that can be used to stick the ***** or other small parts, during assembly.
I had a 400 some decades ago from a '67 GTO, it ran over 300k miles, many/most of them towing cars, and boats....and it would still hit 3rd pretty good....it was subject of much humor wit the tranny guy who built it, when after 15 years of this abuse I said it had to be some sort of extended warranty, because it didn't chirp rubber into 3rd anymore.....
I used the B&M kit after looking through the kits. They all seem to do the same mods. I built it to the st/str settings. I noticed at low rpm- idle shifting 1-2 clunks. Normal throttle is fine and it is firmer then stock but not neck snapping. I used the TransKit overhaul kit.
I installed the B&M kit with the "race" settings and it has worked well. I have run the living **## out of my TH400 on the street and at the strip and it is still running strong. I has been very hot (over 270F at the cooler). Everything else is stock (conveter is 9.5" billet), with an unknown amount of miles. I would use a good synthetic fluid. I have had good luck with Redline High Temp ATF.
I also have a '73 with a TH400 - I installed the B&M with the street setting and have had similar results as others have described. No problems whatsoever, nice firm shifts when you put your foot into the gas pedal.