I'm sure there's some benefit in undercar bracing for the C34; but I don't have one to evaluate or measure off, so that makes it difficult for me.
I know there were guys using R33 GTR undercar bracing on Stageas while back; it might be worth looking down that route?
I elected to use steel rather than alloy, because it's definitely my favoured medium; I know I can get excellent strength and good quality welds. Whereas with alloy on a structural member; I'd like to be heat treating them after welding, and that would make them stupidly expensive.
Also, on the brace I'm making, due to the thickness of the alloy required to be strong enough; my thin wall CrMo tube with steel ends is basically the same weight; possibly even lighter. You could virtually jack the car off it; the stuff is so rigid. I wouldn't bet on the same for the alloy one.
As Iain has said; tying together the radius rod mounts could definitely pay dividends, and it is a very common mod.