Headers will make the NA motor more of a dog down low because they don't have forced induction to make up for it.

I'd just go with a 3in downpipe and a plog on a NA motor, no real need for headers. And you'll never ever outflow the stock catback being NA as they can handle 350whp pretty easy. You might make 180whp with what you got going, add a GT2 cam and it might bump you up 10-15 more, then you're at stock gtp power.