Before adjusting anything, make sure the U-bolts holding the driveshaft to the pinion yoke are not overtightened. Correct torque is 17 foot pounds and no more. In following this set up routine you may find parts of the car where the drive shaft should go. While this may require some redesign, it is better to learn this before the car is complete and is difficult to correct. A length of 2x4, or similar material, can be installed between the transmission and the rear end pinion to check angles and to determine if any chassis or body components will interfere with the drive shaft.
Various methods are available to adjust for proper pinion angle.
On a parallel leaf spring, wedge shaped shims can be added between the spring and spring pad. On triangulated 4-link, such as this, the upper and lower links can be lengthened or shortened as required.