Gareth, why do you think you need a rear sway bar,and, a stiffer rear sway bar?
When racing, i ran a standard rear bar, and a thicker front bar, and would adjust the stiffness on front, by moving the drop links fore/aft,
The rear, by simply backing off the nylock nuts, it would reduce prpensity to oversteer, and also allow the car to "flop over" before bar started to work, ( good for wet tracks)
Spring were 250-275 front, 225-250 rear, a stiffer front, say 300-325 would have been preferable, as the front bars drop links could have been backed off a bit more if needed.
We cant easily fit blade adjusters to the bars in a Z, unless its custom fabricated, but unless you are after the nth degree, why would you need them?