I cut the stock rear perches off and moved them to the top of the axle. I set them of top of the axle then jacked it up to where it was just touching the springs. I then measured from the backing plate to each perch and moved the axle around until they were even. Then I roughly set the pinion angle by eye. A quick tack weld to keep things from moving around and then I jacked the back up to get the weight on it so I can be sure I had the angle right. After I was sure I burned them in.
The front was a bit more complicated, but not much. I got new perches since the driver's side was originally cast into the pumpkin. Because of the extra material on the drivers side I had to modify the perch to sit even with the passenger side, basically I just used a torch and opened the cutout larger and deeper so it would sit down on the casting. I made the mistake of aligning the new perches parallel to the old ones, moved in to the new locations the same way the rear was done, only to find out that my caster angle was off. A couple degree shims are in place right now until I can get the time to drop the axle and move the perches.
If you are going SUA the front perches are about 1" to far apart for the YJ springs so a little modifying is all it would take to get it to sit right in place. For the rear I think its about the same, but it only takes a few minutes to cut off the old perches and relocate them to their new position and re-weld. You would probably want to do this anyway to change the pinion angle.
90 YJ 5.2 Magnum, AX-15, 231 SYE'd
D44 front, welded/trussed AMC20HD rear. 4.56 gears soonish.
Western 6 lug aluminum wheels, with 35x12.50x15 Wrangler MTRs
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