offset shackles or into the frame-rail??

I put the offset hangers on my 70 340 duster. they gave a little more clearance which is what I needed and sounds like what you need too. To take advantage of the kit you put in the frame, you would also need to mini tub or tub it.
There is one issue with the offset front spring hangers that was kinda of surprise is that you have to cut a hole in the frame rail to allow for the welded on nut to fit. the nut is for the main spring bolt. One trick I discovered is to put the 4 serrated studs, for the hanger to frame mount, in after you put the hanger in place. You won't need as big of hole for the nut. I hope this isn't too confusing.
The other issue that is goofy with the mopar spring perches is the hole is much bigger than the leaf spring bolt head. I think I tack welded a piece of 3/8" tubing into the perch to compesate for this.