More HP on 273 Stock 63 Dart GT Vert?

If you're hunting down the original 1965 cast four barrel manifold the part number is 2465726.

Numbers I have recorded from some old notes from an Offenhauser catalog are:
Offenhauser 360deg Equaflow - 5699
Ofenhauser single high rise - 5748
Offenhauser 360 Deg Dual Port - 6022-DP

Offy also made a dual Quad manifold for the early 273.