I wouldn't be concerned about the strength of the standard castor/tension rod unless you are intending to rally the car. They simply hold the lower control arm in tension. Any shear force present in that rod is negligible.

On my coupe, I simply had my standard 1200 castor/tension rods threaded and used a nut on each side of the mount. Must have been like that for a good five years including alot of bouncing off ripple strips, the odd excursion to the infeild/outfeild here and there, and a couple of incidents with a bank at Mt Cotton which tore a castor rod mount off the chassis, and a kerb on a road rally which sent the car airborne. Neither damaged the tension rods.

No sense adding weight where you don't need to.