Thanks for the replies. The current distance(pinion to flange) on my '91 is 63.5" w/only 1.5" shackle lift. With the dual cases I am looking at about 57" right now, more after I finish the rear lift. From the few I was able to measure at the wrecking yard they are a few inches short.
Since I haven't found a definitive reason for not running the 2pc, I think I am going to cut them down first and see how they run. I think the most likely reason why most don't run the 2pc and dual cases is that they have already replaced the d-line by the time they get around to running the dual cases.