You can run a Watt's link or even a panhard on a leaf car.
I have a panhard bar on my AMX. Works great. Doesn't upset anything, some people have said that it would on a leaf spring car.
So how much will either help rear roll?
Rear roll is controlled by spring rate, and anti-sway bar rate, with properly matched shocks. With the tire air-pressure, side wall height and stiffness having some effect, which also works with the shocks. The panhard bar, and Watts links are mostly for side to side control, neither has any real up and down control of the vehicle. Keeping the car's center-line and rearend's center-line in line.
Tom
"Everyone should believe in something; I believe I'll go fishing."
-Henry David Thoreau
Men and fish are alike. They both get into trouble when they open their mouths
