I have done bastard pack with S-10, Dakota and Bronco II springs. They all flexe well AFTER I did a lot of leaf swapping and mixing. You get out of these bastard packs what you put into it. The Bronco II is my favorite for the following reasons:
1) It gives 4.5" lift with a 1.5" lift shackle. The shackle is always citical to getting good flex. I have 13 inches of travel with it split fairly even. I ran an RTI ramp and got a 654 on a 30 degree ramp, shock limited. I run 31's and they stuff real well.
2) You use the whole pack including the main leaf under the XJ main so there is no messing around with swapping leafs to get it to flex well.
3)The leaves are thin and the rub pads are retained. Thinner than the Dakota or S-10 leaves.
4) I uses a 3/8 center pin. I have destroyed several stock center pins (5/16?). I now upgrade to 7/16ths center pins which are a snug fit in the Bronco II pack. Still have to drill out the hole on the XJ leaf.
5) And best of all in my view is that the Bronco II spring clamp (one per pack) is riveted in place. You unfold the ears and use them to located the leaves laterally. Now the leaves are not bound up when you unload the rear and they just keep on dropping. I could probably get 14 or 15 inches of travel out of this set up if I couild find the right shock. The clamp is located in the front of the spring and is about even with the rim. It wont rub on the tires and you can run stock offset rims with 10.5's and have plenty of clearence.
6) The only negative (because I am detail oriented) is that you should/must use a teflon or HDPE liner between the XJ main and the cut down Bronco II main to get maximum flex under compression with out huge loads. This is true for all bastard pack buildups where you use the main leaf from the donor pack (after cutting off the eyes). I polish the XJ main and then bond the liner on (HDPE) using weld wood adhesive.
John
