From my understanding, many exhaust manifolds had different flow rates and THOSE engines seemed to run fine. Take a look at just about any performance Mopar setup: The big block Magnums for one. The left side surely seems to be less restrictive than the right. The 340 manifolds look the opposite to me. The right side of the 68-71 models seems like a cast iron header while the left looks like a larger volume 318 piece.