Dual exhaust off the stock manifolds,is what I suggest.
Headers with a 318 are not really worth a lot of power,
especially if the 318 is stock compression, etc....
I did put a cam in my 318, Mopar Performance # 4452759.
http://store.summitracing.com/partdetail.asp?part=DCC-4452759&autoview=sku
Basic Operating RPM Range: 1,200-5,200
Great for the street. :)
Listen to the car, with the 318, cam, and dual exhaust.
http://www.youtube.com/watch?v=Ud8mci4rjFU