Installing a Getrag 240 into an Opel requires using an automatic driveshaft as the yoke differs from the 4-speed driveshaft yoke. These driveshafts are getting rare.
Now, while there are GM Yokes with the same diameter and number of splines as the Getrag's output shaft their use is not recommended as there is one major difference!
The GM splines are straight cut while the Getrag are beveled. Using the GM yoke could cause the binding of the yoke on the output shaft.