I'd get a full spring over an add-a-leaf.
Without air tools putting them on an old XJ can be a bit of work, possibly hassle.
If going to do that much, might as well just do it right the first time.
Not made of money, but busting my ass (knuckles and other) working on an older rig put together with top notch Chrysler fasteners............my time is worth something.
I helped a buddy add a leaf without taking the main leaf off the jeep. Big heavy duty c clamps are your friend.