Header advice needed!

Hey all! Looking to get some oomph out of my 1967 273 in my dart GT vert. Planning on hopefully having it rebuilt to the 1966 275 hp specs or better. (In a perfect world I would love to get 300 hp, but prolly not likely out of my old 273)
Looking to replace the stock exhaust manifolds with headers, and considering these. Any thoughts on these, or what are my other best options?

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