Performance loss from headers to HP manifolds

I think manifolds would be fine..And that 268 cam should clear the pistons by a mile, even with no valve releifs. But, running NOS thru it will really restrict things. If it were me, I'd run the manifolds without flinching a bit, but I'd upgrade to a 2.5" exh system. The XEs extra duration will help pump the exh thru the manifolds. I dont think you'll lose much unless you consider the choking from NOS use. And all that means is instead of 125 ish HP, the kit will make 100. Not a big deal IMO.