hi guys, I am swapping my th350 to a th400 in my 1976 olds cutlass. how could I know precisely how much I have to shorten my driveshaft ?

 

With the rear supported by jack stands, slide the trans yoke in all the way and then pull it out 3/4 of an inch. Measure from the center of the rear pinion (the flat part) to the center of the front yoke ujoint hole.

 

Talk to your driveline shop. Each one measures slightly differently and you need to know what they require to get it right.