Be careful - even 1/8" difference in diameter is a big deal. Too big in front just makes it plow, but too big in back will loop it. You know the saying "Oversteer is when the passenger is scared, understeer is when the driver is scared").
If you find one that fits but looks too heavy, you can reduce the rate by lengthening the arms.
Factors that increase stiffness:
bigger diameter
shorter center distance
shorter arms
links to the suspension closer to the chassis center
