I've used three different sets of the Mopar kits on two B bodys and one A body. I have had them installed (I'm not a good welder, yet
) with the outside edge welded into the outer subframes to get max inset, the first set in a B bodys was centered like Mopar shows them and the last set in my 1969 Dart GTS is welded so the inside of the box is welded into the inside of the subframes to get better leaf spring clearances on the passenger side to the stock gas tank
We rarely cut out the outside of the frame rail, which moves the springs in the max amount. Granted, with the spring not being centered under the frame rail, you need to use offset shackles, which is easy enough to do. I probably haven't installed an actual "box" in the last 10 or so I've done - I simply shear 1/8" plate to weld in for the top and front, and trim a new side plate to mirror the outside frame rail. It isn't an issue with stock style bushings that have an inner liner to tighten the front bolt up solid, but with springs that run a solid front bushing, (and you leave the bolt loose and lock-nut it) I weld an 1/8" washer or plate to the sides to keep from beating the holes out of round.
