I disagree with the pick a manufacturer any manufacturer approach. They all make unique headers with different bends, sizes, and styles.
I'd ask the guy if he'd mind letting you see if you can drop the motor in with the headers on, and if they don't fit, get a refund. However, if you're not ready to drop a motor in, I'd just hold off and make sure I got the right ones the first time, unless of course you're getting them on the cheap.