I bought a 455 with a TH400 about a month and a half ago. Before I bought it I was aware that I needed a short tailshaft TH400 but this transmission I have has been rebuild recently and I got a real deal on this. So my question is, how much do I need to shorten my driveshaft to accommodate for the long tail shaft Th400?

Also its going in a 65 F85

 

The crossmember's transmission mounting point needs do be moved back 5 11/16" to use a th400 If it is currently setup for a th350.

As far as driveshaft length, that depends on your yoke sizes.

Once the trans is in the proper place, measure from the back of the tailshaft, to the front of your pinion yoke.

 

Great thanks man!

 

When you measure your drive shaft make sure the cars weight is on the axles. Put your jack stands under the rear axle not on the frame. When you measure subtract 3/4" for shaft play.

 

Each drive shaft makes will have his own perfered method of measuring length. I'd call your maker of choice before taking it in to be shortened.

 

I bought a long tail TH400 and swapped it with the tail off the old trans so the mounts would match up Do you have the original TH400 for the car? It would be easier and cheaper to use your original tail and no shortening of the drive shaft

 

Nah I don't have to original trans, it was a jetway but now its a TH350. So wait, I can change the tail on the TH400? I mean how hard is it actually to do?

 

Well I bought a long tail TH400 which is what I needed but the mounting bracket holes where in the wrong place so I had to use the long tail off of my old one. I'm not sure if the short tail is all one housing or if you can take it off like the long tail I'm thinking you might be able to put a short tail on by swapping them anyone know for sure? Swapping my 2 long tails was very easy. 10 minutes when it was already out.

 

It's not just the housing, but the output shaft that needs to be changed if you are going to do that. If you don't know what you're doing don't try yourself. A shop could do it easily enough, but I don't know what they'd charge for that.

 

I just changed from a long tail 400 in my 70 cutlass to a short tail 400.
once bolted up took the measurement from the flat of the yoke at the rearend where the U-bolts pass thru to the end of trans spline and took the old driveshaft and he loved that measurement.as said before check with who will be cutting the shaft to see what he/she prefersyou will be able to do it a lot cheaper as yours will need shortened about 5" as mine had to be retubed as it needed another 5"don't quote me on the 5" as that was just a guesstimate