Heres how I do them. Take out the old rear end and measure the distance between the centre bolts on the rear springs (take both measurements from the left side of the bolt will give you dead centre) draw a line at this distance past the pad area directly on the spring. measure the same distance on the new rear end and grind off the welds holding the old spring pads on if they are not the same distance making sure you dont dig into the axle tube itself. Next take the Chevy spring pads off the old rear end without digging into them, tidy them up and sit them back on the spring . With help put the new rear on to the old spring pads and measure and check the distance marked before. Measure from this point to the back of the backing plate and shift the rear axle until the measurements are identical on both sides, check the position on the chassis as well . Next you need to set the angle of the diff. you need to put the car on stands under the springs at the back and lift the front end and put it on stands as well so it is level. Hire or borrow an angle guage and set the pinion to 3 degrees. recheck ALL your measurements and TACK weld one side, measure again and do the other side. Then take the whole axle to a welder who knows that you have to be careful welding the assembly so it doesn't warp. A new tailshaft, adapt the handbrake and the rear brakes making sure the wheel cylinders match your system and you are done. Easy huh !