extending the rear shackle mounts does 2 things:
1) raises the ride height
2) changes the diff pinion angle

the latter can alter the way the car puts power down. But it is better done with wedges fitted between the diff and springs. You will need an expert to measure the diff pinion angle to determine if you need to alter it. Remember also that under power the diff pinion angle will change - it will rotate upward at the front of the pinion due to torque reaction - how far depends on your springs and anything else you have on the back. one of the things that 4links and tramp rods do is help to minimise dynamic changes in pinion angle.