For a 318? Hooker competition series 1 5/8 and get them ceramic coated. Best bang for the buck on a 318 header.
Then use 4 turn downs to make it past the torsion bar cross member These shown are 3 1/2 pipes but you can get the picture. Use any Diameter you want.
Put a turndown on the header and face it in and face another the opposite and tack it. You can move them to level them. That S on each side is then one piece. I ran 3 1/2 out the back from there using the same method. Cheapest mandrel bent exhaust you'll get. It sounded good and went 10.50's through the mufflers. It had 2 inch tubes with 3 1/2 exhaust on the orange car but like I said you can make your own size.
I did 1 5/8 with 2 1/2 on the brown Duster with a 318 worked great