Might want to look for some good used Dougs or TTI's.
Have you considered 340/360 cast iron manifolds. They are the most street friendly and worry free.
For a 73/74 left casting (like 68-70 but with heat stove) and 68-70 passenger casting I'd think about $250-300.
What's the cam, intake, heads, and carb in your 360 current motor? Might not be something that you are getting a big gain by running headers?
