Moving rear end back

I am doing it to gain additional tire clearance at the front of the wheel opening in order to use a larger O.D. tire. I had a friend of mine make me some 1/2" spacers but when I went to put them in with the new offset mounts I bought, the studs are too short to get the nuts on fully. I would have liked to have moved it back more than 1/2" anyway. I have several longer driveshafts to use if I move it back far enough to require a longer driveshaft so that is not a problem. I can't say right now what the angle of the rear shackles are as the rearend and springs are out of the car. I am just curious about the safety of using the other set of holes in the spring perches.