I have a 350 turbo in my car and I was wanting to put in a 400. I all ready have the adapter, I had to buy it for the swap from a 5speed to the 350. Was wondering if I got a 400 with a long tailshaft if it will bolt right in are if I need to do anything else.

 

Switching from a 350 to a 400, I can think of a couple things off the top of my head. First the yoke on the 400 is a larger fine spline so you will have to find a 400 turbo yoke. Also a short tail 400 is still slightly longer than a short tail 350 so you will have to shorten the driveshaft slightly, if you install a long tail 400 and had a short tail 350,,,you will have to shorten the driveshaft dramatically. Even the short tail 400 will require the cross member to be moved back and inch or two over a short tail 350.
Also the kick down mechanism on a 400 is electrical, you will have to wire the switch to either the carb or the gas pedal, GM had them both ways in the 60's and 70's, if you run a manual valve body then that is no concern.
The 350 has a cable mechanism for the kick down (passing gear) Not sure how you had that hooked up. Some people run with out it but you don't build the line pressure necessary at full throttle, but with a manual valve body it doesn't matter anyway.
No 400 turbo's had a provision for a torque arm mount either. Some 350 turbo's did though. So if you were using a 350 with a tailshaft that has the factory torque arm mount, you wont have that option with a 400 turbo. Aftermarket torque arm setup that uses aftermarket cross member for the mount is the only thing I can think of, you may already have this. But like I said, the cross member will have to move back an inch or two for a 400 turbo and this may give you torque arm problems, not sure though as I have not looked at the aftermarket torque arm setup closely. These are just a couple things off the top of my head that I can think of, mainly from messing with older 60's and 70's cars, there may be some more unique instances for these 3rd gens. I know there has to be some people on here that are running 400 turbos, maybe they will chime in here and can give you more in depth installation help, hope this helps, Larry.

 

The tailshaft is long and all I had to buy is the torque arm bracket and thats it for the 350 and it bolted right up. Im wanting to know if the 400 will do the same if I find a long tailshaft 400.

 

I am not sure what kind of torque arm bracket you have. 350 turbos had a torque arm mount molded into there tail shaft housing on certain models but those are hard to find. 400's never had a tail shaft housing like that so you must be referring to something aftermarket. If it bolts somehow to the 4 bolts that hold the tail shaft housing on then I guess it would work, not real sure if the bolt spacing is different on a 350 to 400 for the tail housing. Do you have a pic? Larry.

 

To answer your question.... NO, it's not that simple.

I have a th-400 in my car, so I been through all this already.

The th-400 torque arm mount that you get, bolts on at 4 points.

Between the trans mount and trans.. 2 bolts

Then the bracket is U cut on the side, with 2 holes. You take the bottom and top tail shaft bolts out of trans, put bracket there and put bolts back.

In away it's shaped like the bracket you have now, just bigger and bolts in a diff place on the th-400.


Next and biggest prob is the crossmember.

Bellhouseing to trans mount on a th-350 is 20-3/8" (long and short shaft has same face to mount spacing)

On a th-400 short shaft it's 26-3/4"
On th-400 longshaft it's 28"

So see if yourng a th-400 short shaft, you will have to move the crossmember back almost 6"...

There's no way to do that on a third gen F body.. Unlike the older cars, the front sub frame ends about 2" past the 1st crossmember bolt.

So you have to a have "th-400 swap crossmember" It's built off set... It bolts to the stock holes in sub frame, but it's off set to the rear almost 6"...

Now the drive shaft lenght...

The th-350 short shaft is 27-5/8" overall
The th-400 short shaft is 28-1/4" overall

But most th-400 slip yokes are longer, so there's a 75% shot you can use the same driveshaft..

When I put the th-400 in mine, I used a driveshaft from an 82 or 83 F body with the 200C 3 speed auto. The shaft fit just fine with my yoke..

Next is the yoke.. You'll have to get a th-400 slip yoke as it's bigger than a th-350

Then next is the front U joint... The th-400 uses a larger yoke, which means a bigger U joint. You will need a conversion U joint. I bought mine at auto zone. One of the brute force forged joints. It took me and the smart counter guy close to an hour digging through boxes to find the right conversion joint.

It has to be the smaller size with inside clips to fit the driveshaft, but the yoke side has to be bigger and outside clips.


For the swap kit, I bought the hurst th-400 swap kit. It came with a round tube type crossmember, ready to bolt in, the TQ arm bracket, vac line, etc.. I think it's $115-125 now from summit.