Astroracer brings up some very good points to consider. Interesting article there but I noticed the author doesn't address narrowing the centerlink. If the centerlink is narrowed in the center the exact same amount as the subframe bumpsteer shouldn't occur (as long as the upper control arm bushing, tie rod joint and lower control arm bushing are in the same plane as engineered by GM).
Camber and caster should be able to be brought into align if the horizontal and vertical planes of the control arm attaching points were not changed from original. I think that is Astroracers point. Changing the subframe width can radically effect geometry.
The article states this suspension will raise the truck 3 - 4 inches higher than stock height, then you'll get dropped spindles and perhaps springs to get the truck a little below stock height.
Just a few more thoughts - This change looks like some skilled fabrication and welding are needed, if you can do that, great, if not that fabrication talent cost's $. The spindles and springs will cost additional. If you haven't already cut the frame have you compared using a Mustang II based unit cost to the costs (spindles, springs, disc brakes) and additional work of this subframe?
I certainly don't want to dissuade anyone from being creative and resourceful in their build but want to assist looking at issues and options.