Moving rear end back

I would like to move the rear end in my Dart back some to gain additional clearance between my tires and the front of the wheel openings. I was going to use spacer plates between the front spring mount and frame but I have heard that you can actually just move the rear end back on the spring perches so that the pin on the spring goes into the forward hole on the perch. Is it really O.K. to do it this way as far as being safe and does it affect the pinion angle at all? Are there any other issues to consider when doing this? I am aware that matching holes will have to be drilled in the shock mounts and I will need a longer driveshaft as well. I would appreciate hearing about the pros and cons of moving the rear end this way from anyone who has done so.