So I 350 rocket swapped my 98 a while ago and I was never happy with the 200r4 so I’m in the middle of swapping that for a th400 and i went to put the driveshaft in and the yolks a different size. What size yolk is needed for a th400 and can I get a custom driveline place to swap the right yolk onto my existing driveshaft?

 

TH400 is a 32 spline yoke. yes, a driveshaft shop can make what you need.

 

The driveshaft will also need to be shortened for the TH400

 

even if it’s a short tailshaft th400?

 

The short tail TH400 is about an inch longer than the 200-4R to the end of the case. Your original driveshaft may not have sufficient engagement on the output shaft.

 

is it as simple as tellin the driveshaft shop to chop an inch off the drive shaft?

 

The right way to do this is to install the trans first. With the rear suspension at ride height (weight on the rear springs), insert the front yoke all the way into the trans, then pull it out 1". Measure the center-to-center distance between the u-joints. That's the dimension they need.