We have seen up to 1" difference driveshaft length in various GN's when it comes to a trans/driveshaft replacement, so it should be measured like Paul stated.
If you are installing a TH400 for track use, it would be smart to replace the u-joints with the 1350 series or you will have a time bomb waiting to go off!
I learned this the hard way years ago after I spit out the D/S on the track twice.
If you do not have enough torque or traction or balls in your car, dis-reguard my comments above.