1970 inline 6 cylinder header

Hi guys, Just purchased a 1970 coupe. All numbers matching, needs a new exhaust manifold. Wondering if I should put a header on, it is a 250 inline 6 cylinder. Saw from Mustangs Unlimited I could put on one with single or a double exhaust outlet. Currently have single exhaust with standard muffler. Want to put dual exhaust with Magnaflow muffler. Should I just replace manifold or change up to header ( if so single or duel outlet )? Will be using her as an everyday driver, but would like some extra horsepower and would love to hear the sound of a sweet exhaust. If I use header with single outlet , should I change muffler to a magnaflow or other high performance muffler? Also is there a lot involved in changing over to dual exhaust? Or would single exhaust be adequate for what I am looking to do? Thank you all so much for any input you could give me.