I just did mine last week. I did it on 2 - 6 ton jack stands placed just forward of the spring eyes and then a 3 ton floor jack under the pumpkin. I did post on this in my thread. These videos should help. Work on one side at a time. Undo your shocks and sway bar link if you have one. TAKE a pic for reference if you're not sure how to put it back together. Indo the U bolt nuts with axle supported. Undo the shackle nuts top and bottom, remove the shackle and lower the spring. The with a 3/4" long socket you can get at the Eye Bolt through the frame. With a 3/4" wrench undo the nut. This is a deformed lock nut, so it will be stiff getting it off. Installing is the reverse, BUT make sure the locate spring pin is fitting into the hole on the axle plate. It may take a bit of wriggling to line it up. Once in, place the U Bolts and loosely tighten the nuts, reconnect the shackle and tighten the nuts. Use new nuts if you're using your old shackles. Oh, I forgot to mention to install the new rubbers (or Urethane ) A little WD40 will make that easier. You will need to adjust the height of the axle as you do this by the way. Replace the shocks and sway bar. One side done, repeat on the other. Once the car is under full weight, tighten the U Bolt nuts to 45 ft/lbs Took me all of 2 hours. Then go drive for 40-50 miles and retorque all the nuts.
For springs, please do not buy Grab-A-Track, I was not happy with the ride height unless you want the rear end way up in the air.
I highly suggest using Eaton springs and as I said, I found the Boss 351 153lb just about perfect for the Mach 1. Also look into larger sway bar and better oil shocks.