Not sure on 5th gens fitting 6th gens to be honest bud....Not to sound like a **** but would there be a need? 6th gens already have strut braces attached to the firewall which are more than good enough at providing rigidity. If it's anything like the EK's and MB6's that run a similiar setup, you will make it worse by replacing the two small supports with one large piece, as you are exposing the struts to more flex under hard cornering.
Imagine a pencil. A very small one is hard to break, where as a long pencil can be snapped with two fingers.