Moving rear springs.....

The 3" spring relocation kit involves notching (cutting) the frame for the M.P. boxes to be welded in the frame for the front spring hangers directions should be included with the kit.
I have seen post where other people have decided to just mount these boxes on the inside of the frame to avoided the cutting. The drawbacks are less room for the exhaust, The gas tank will be in the way of the springs, and the spring perches will be even closer together giving the car less stability around corners.
I would not recommend it, spend the money and do it right either way you still have to relocate and weld the perches on the rear end so a welder is still required