Yeah, that is pushing it with SOHC's. The Cobras already have forged crankshaft and a little bit more beefier bottom end to begin with. Probably 450HP with 500HP being the "Pushing it". I'm throwing 580HP down on the rear, trapping at 121MPH low 12's and that's just 'playing' with it. I don't launch hard yet as I'm waiting to get some subframe connectors and roll bar installed and some new control arms. When said and done with slicks I'm confident I can hit high 10's at 130+MPH.
410-430 is safe with them. Anything higher will cause problems down the line when it comes to internal integrity....especially the hyperpathetic pistons.
My car ran 12.53@116MPH. That is with Nitto's at 22psi, and leaving at idle. I have never launched the car. 414RWHP. I want to install meth, and it will be around 500 safe RWHP.