273 with 340 exhaust manifolds

From the lack of info, you can't tell if the 340 manifold could make 10-20-30 HP more with other parts changed in the engine. In other words, the info is not about the manifolds by themselves, so seems to be meaningless as a fixed number. Isolated case info is not the same as generally applicable data. But hey, I just put headers on anyway! I 'butt-dyno' know how much my 351C lost when going from cheap headers back to stock manifolds....