Brian-
The first thing to keep in mind is that the tolerances on these trucks would make any A-type personality cry. From the factory, there are many tolerances that are plus or minus 1"! They built these trucks for work, and weren't too concerned with everything coming out exact. I don't like to use this as an excuse, but it's reality.
As mentioned, we have the two options:
1. Use the new rear crossmembers I sent you and it will set the chassis to the same dimensions as the ones I have in the shop. I sent the center to center bed mount measurements for you to compare to your chassis, and get a better idea if this will be the easiest route to go. If your center to center on the rear bed mount hole in the chassis will work with the measurement I sent... good to go. It was taken off a 67-72 bedfloor from Old Aux. The cab mounts will be of no concern, as the trailing arm x-member fit just fine.
2. Second option is for us to make a 1/2 wider upper shock bar. We will happily do this, as we always guarantee all our products for both proper fitment, and customer satisfcation.
If it were my truck, I would verify the bed mount holes, and if it were close.... I would slap all the new tubular crossmember in place that I sent you, and call it a day.
We have had a handful of guys that have already performed this same conversion on a leaf sprung GMC... without any fitment concerns. It migth just be your truck that is off a bit more than others.
Let me know what you decide, and we will do whatever we need to.
Nate