Transmission swap

[kalle]

I have 71 Nova. I have recently swapped the small block for a big block. I have changed the heater box and the frame mounts and I have modified the 350 cross member. The chance has come up of buying a th 400. What modification must I do to make it fit? Must the propeller shaft be shortened? And if so, how much? Must I replace the tranny cross member? I already have a B&M shifter so shifting gears won’t be a problem.

Or, maybe should I stick with 350? What do you think, what is best on a street car which sometimes goes to the strip? It is better to make my 350 stronger? The car has a 454.

Thanks for your inputs!
/// Kalle

[Maxturbo]

It will depend on which TH-400 you get. There are at least three dif tail housing lengths that will dictate the length of drive shaft you need. Also the drive shaft yoke will have to be changed as the spline portion is different as well. Make sure that it's a CHEVY TH-400. The Olds, Pontiac, Caddy and Buick versions have a dif engine bolt pattern. ;) On the cross member...you may have to modify your existing or buy an aftermarket member made for the 400 (in a Nova). I think J&W sells one.

[68chevyll]

As long as you are not going to beat the snot out of it a 350 is fine. Later.


Mark!

[kalle]

Ok, i know this an old thread, but i thought it was the best way get some answers.

It will depend on which TH-400 you get. There are at least three dif tail housing lengths that will dictate the length of drive shaft you need. Also the drive shaft yoke will have to be changed as the spline portion is different as well. Make sure that it's a CHEVY TH-400. The Olds, Pontiac, Caddy and Buick versions have a dif engine bolt pattern. ;) On the cross member...you may have to modify your existing or buy an aftermarket member made for the 400 (in a Nova). I think J&W sells one.

Which is the best to use? Is it possible to measure length or identify the th400, so i can find the right one? How much must I shorten the propshaft?
What I believe it is not hard to fins a aftermarket crossmember, so that’s easy.

Kalle

[bowtie0069]

Is it possible to measure length or identify the th400, so i can find the right one? How much must I shorten the propshaft?


When I switched from a TH350 to a TH400, the one I found was the same length so no issues there. Check the overall length of both and see if you get lucky. You do have to change the shaft's yoke because the splines are different, but I never shortened the shaft. The crossmember does need to be changed too.

[1quik69]

You need to be sure that you get the Chevy TH400. The trans you need would be the short tailshaft version. If you look at the trans the tailshaft housing will only be 4" long. The tailshaft housing bolts to the main body of the trans. GM made a short and long tailshaft version of this trans. The long version went into full size cars etc.

[kalle]

So, lets sum this up:

I need to find th 400 “shorty style”, 4 in tail housing. I don’t have to shorten the driveshaft, but I have to replace the u-joint and the yoke in the tranny end of the driveshaft. Is this correct? And of course a need a th400 crossmember, and a chevy tarnny.

I founded this on hotrods page: http://www.hotrod.com/techarticles/identifying_gm_automatic_transmissions/photo_04.html

It is a big difference between “rear transmission mount position” numbers between th 350 and th 400. Can anyone explain?

Kalle