There are several good ways already shown. The lower arms need bracing forward and back and a bonus is if the hub carriers are too. The lower arms can be braced and also bridge the weld with something like this.
Jaguar Rear Lower Arm Gussets Jaguar Rear Lower Arm Gussets [Jaguar Contemporary CCX] - $80.00 : Acton Custom Enterprises, Custom Metal for Cobras
I have also seen tubular braces added to these arms instead.
I have also seen urethane bushed radius rods in the fore aft position. Yet another option is to add radius rods to the front/rear or both of the lower arms so they theoretically don't change length.
It's a bolt on attempt to make a tubular boxed lower arm.
I like the function of the Unique/Arntz brace but not the look of the size and the mounting hanging below the car.
I would do something to beef up the arm if I wasn't going to do anything else. Anything in addition or instead is a bonus.