Maybe a few of us should do a column on properly setting up a spring over. I have more experience on the CJ side, so I would need some help out of the YJ guys.
I think you can get away with not using a transfer case drop if you do the slip yoke eliminator and a CV shaft in the rear.
Other than that, pick a starting point, front or rear. We'll say rear to begin with.
Support the vehicle first of all. Remove all your u-bolts, shocks, rear trac bar and drive shaft. Take loose the shackle end of the springs and pull the axle out. Measure the distance between the perches or measure the distance from the perch to the brake backing plate, whatever point of reference you want. Just make sure you know where they will be positioned when you put the new ones on top. Now bolt the springs back into the shackles. Find a c-clamp and clamp it on the spring near the centering pin. Remove the center pin and turn it over when you put it back in. This is what keeps the spring centered in the perch. Cut the old perches off and gring the old welds down smooth. Roll the axle back under there and place the new perches on top of the axle. You can go back and weld them later. Put all of your u-bolts and spring plates back on loosely. Point the pinion yoke directly at transfer case yoke. Tighten the u-bolts down snug but not real tight.
Lower the Jeep down onto the springs and remove the jackstands. You will notice that the springs are probably flatter than they were before. This is normal and they will get flatter over time. Now look and see where the pinion is pointed. It has probably move upward since you let the vehicle down. Now you want to set the pinion angle. You can so this with a cheap angle gauge from harbor freight, but since it will settle anyway, I would eyeball it. I would point the pinion directly at the transfer case yoke again then lower it 5 degrees from there. It is supposed to be 3 degrees or so, but like I said, your springs will settle over time and the 5 degrees will turn in to 3 degrees or so. Check your perches against the measurements you took earlier and tighten the u-bolts. You will have to make a new bracket for the track bar to make it taller. This is where the YJ guys should chime in, but I would say measure the diameter of your axle tube and thickness of the spring pack and that should be the additional height you need for your track bar bracket.
You will have to do some measuring for the new driveshaft and shocks, then get an extended brake line for the rear.
Don't forget to go back and weld the perches once the front is done and you have triple checked your perch spacing and pinion angle.
Except for the finite details, that covers the rear. I'll let one of you YJ guys cover the front. I can't recall if the perch is incorporated in the casting on the front differential or not. If so this will require some grinding to mount the front perches on the top.
