I am wondering if a driveshaft from a 9" rearend will work for a 8" rearend.

 

It 'should' be the same length, my 8" appeared to be the same distance to the snout as my 9" that replaced it, the diameter of the 3rd member is just larger around. There are a lot of variables like what year 9", etc. that may cause a 'your mileage may vary'.
Jon

 

It will all depend on how much further forward your yoke sits. Different style 9 inches had different yokes. If anything your drive shaft will be too long. You can take it to a shop and have it shortened and rebalanced. It isn't anything out of the ordinary.

 

of course it could be used, but as above the length might be wrong. Measure U-joint center to u-joint center. you might have to replace the U-jouits, but that's something I'd do anyway with a used driveshaft. of course the slip yoke might be different, so you might have to get a differnt one if you don't have a fitting one you could mount already

 

All other things being equal, the 9" will require a shaft about 1" shorter than an 8", because the 9" is more bulky, projecting farther forward.

 

The reason I asked is because my friend is giving me his 9" driveshaft from his 67 cougar for my 5spd conversion. He put all new u-joints, new shaft, and it has the t5 yoke on it. If all I have to do is shorten it, that don't matter cause I will have to on the one I have now. Just want to make sure It will bolt up after I shorten it to the right length. Thanks.

 

Way too many variables to provide an answer. You are going to need to test fit the shaft and do the center to center measurements as Kalli stated.

Be aware that the end of the shaft may not have the same end (different size u-joint) you will want to check this and address this if you need to modify the shaft.

Be advised that any measurements need to be made with the rear suspension "loaded" as the distance between the back of the tranny and the yoke of the rear end tends to get shorter as the suspension is compressed.

You do not want the slip yoke to bottom out in the tranny, ever. nor do you want the slip yoke to be pulled too far out of the tranny.

I have found that about an inch of space at the front of the slip yoke is about right on a car like this at ride height.


Additionally, in the past I have sometimes been able to use longer or shorter slip yokes in order avoid modifying shafts.

 

Oops. Cougars have a longer wheelbase than Mustangs, which makes the driveshaft longer. It'll have to be cut.