In trying to decide what parts to use I figured out there is caster to be gained with the newer crossmember. The upper control arms mount further back on the squarebody crossmember vs the older one. I'm comparing '69 to a '91 suburban here. So there's your caster, unless of course the locating pin for lower arms isn't also moved back on the newer x member!
Here's what I'm talking about. Maybe ignore the exact #'s on x member weight differences, I was going by memory. Older x member really closer to 8.5 lbs lighter. The 4 older control arms also about 17# lighter than newer ones.