Finally decided on a cam for teen SB. Should I bother with header?

This is just a IMO comment, but, At a basic look and level I think that 320 maybe a little high but not a way over the top est. the amount of head and manifd porting would play a roll.

Regulardless, the question has not been answered by me. I'll just agree with the above. They will not be worth the time and money since your efforts will not deliver any real noticeable gain via the Butt dyno or anything else you can feel.
As a '73 Cuda owner myself, your header choices are;

Doug's - ....................... 1-5/8
TTI................................ 1-5/8 - 1-3/4
Hooker Super Comp's....... 1-3/4

All other headers are the 1 size fits all style and are not worth spit.
The Dougs would be most in the ball park of where your engine performs.
I would not entertain the thoughts of headers on your engine until a camshaft size of a 220 @'.050 or bigger. And then I'd use the headers by Doug. A good way to finish off is with a header back system / kit by Summit racing or Jegs. They come with and without mufflers at very reasonable cost. They fit my Duster pretty good and require only minor trimming at the tail pipe.