With a single ball joint, load would be distributed evenly across the die regaurdless of horizontal alignment. With a heavy/robust construction, lateral forces caused by misalignment would be negated by the inability to move in that direction.
How much lateral force are we talking about? I bet you could center it up to within .1mm. With that and a reasonable horizontal alignment (no need for a Zig-Align thingy) lateral forces would be really tiny.
C-clamp guts are too tiny and would flex to one side due to said lateral forces. A better solution would be to use 1" - 1.5" .
acme threaded rod (same thread style a C-clamp uses) and round off one end in a lathe. Mill out a hemisphere in a piece of steel, heat it up cherry red, dunk it in water to temper it.
Another advantage to a robust build is repeatability. How many times is this thing going to get cycled? Alot. Do you really want a rig that is going to flex under a 100lbs of force? No. If it is built light, how many cycles is it going to handle before it is seriously jacked up.
As I understand it, what matters in a test bench is a reasonable degree of precision, accuracy, and repeatability. Can repeatability be achieved using screws in the corners without spending bloody ages doing it?
Once things are set up, I bet you could unmount and remount in fifteen minutes using a single ball joint rig, and have repeatability.