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I got a question. I have a 69 grand prix with the 400 tranny..The current tranny only goes in reverse so im gonna swap it out with another one but my tranny a short tail and i have one with 220,000 miles on it that was rebuilt 5 years ago out of a 70 skylark and it a 400 also but the other day my friend pulled a 455 olds motor out of a 98 with only 48,000 original miles and gave me the tranny which is also a 400 but it a long tail..Now i have a tranny jack and have done tranny swaps by myself before so i wonder cant i swap the tail housing from the short tail to the long tail to make it a short tail for my Grand Prix so i wont have to change the drive shaft to fit the long tail..is it possible for a novice like me or it cant be done? Im going have the bad 400 tranny rebuilt maybe next year but in the mean time want to install a new one asap..
 

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The longtail can be changed over to a shortail. But both the tail housing AND the shaft must be changed.

My trans guy changed it on the last TH400 he built for me. I suppose you COULD do it yourself. I once replaced some clutches & swapped in a 34 element sprag. i went by the info in the '69 Pontiac service manual. There was no internet back then. I'd guess that you can find all sorts of DIY YouTube videos, as well as lots of written articles, for TH400 rebuilds.

https://www.google.com/search?q=how+to+change+a+th400+from+long+tail+to+short+tail&rlz=1CAHKDC_enUS777US777&oq=how+to+change+a+th400+from+long+tail+to+short+tail&aqs=chrome..69i57j69i64.32085j1j4&sourceid=chrome&ie=UTF-8

https://www.google.com/search?q=how+to+rebuild+a+th400+transmission&rlz=1CAHKDC_enUS777US777&oq=how+to+rebuild+a+th400&aqs=chrome.1.69i57j0l5.14709j1j9&sourceid=chrome&ie=UTF-8

BUT, unless you just want the experience, I recommend letting an experienced trans guy do it for you. I've found that there are usually some drag racers who builds trans for other racers, who will do it a lot cheaper than a commercial trans shop. I'd ask around to some of your area racers, to find a good builder in your area.
 

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What BigD said. Total teardown necessary to replace the mainshaft. These transmissions are not that complicated, I've done a few, but do require a couple of special tools and good technique installing the seals.
 
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