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

IMO your hp and torque numbers are overly optimistic. But I could be wrong.
I would stick with the manifolds. The magnums flow well and headers will only give you a 10 to 20 horsepower increase at most at your performance level. As you noted the cost will be around $700 for a nice set of TTI's. The cost per horsepower is high, unless the money is not an issue with you.