headers for a 318

People usually put headers on to maximize future upgrades (more power equals more exhaust) but headers and duals will add a bit of a mileage and performance bump which is always nice even just in casual driving and won't make your car into a racer. But headers don't always give trouble free service.

Well said. And sooner or later, a 4bbl set-up will come available at a killer price, that you won't be able to refuse; and you are ready for it.
But even with the 2bbl and a x-over, fuel mileage gains are the norm, as the engine picks up a few more footpounds of torque, largely due to the long-tube headers.
When I had to remove my crossover to fit the GearVender, I really, really, noticed the loss of torque, down low, right in the street driving zone of idle to mid-2000.The TTIs had a simple H-crossover, but it mustabin working really well. I spent the rest of the summer, trying to get the torque back.I was only partially able to do so. The best solution for me,was more rear gear. You probably don't want to go there.

And then there's the Trebleg option.
If you go with his manifolds,(For you, an excellent option),and for your stated usage,a big single exhaust could be considered.Save some money.
So that could be;Trebleg logs, a big single exhaust, and a free-flowing muffler. Just like the 273s of olden days.