First of all, what kind of Wagoneer? There are XJ Wagoneers and also Full Size Jeeps that are Wagoneers and Grand Wagoneers. The stock rear spring packs from a full size wagoneer are longer, I'm not sure exacly how much but I'm estimating around 5" longer.
You would be better off getting Comanche rear leafs, they are spring under stock. A stock pair of Commanche leafs mounted Spring Over will give you around 5" of lift, plus the longer spring will provide more flex. You would have to relocate your rear shackle hanger to accomodate the longer MJ spring.