Think about it........ The standard one was no doubt designed that way(eg in the middle) for a reason, Don't get me wrong on our RA40 3sgte race car we have it from the front, but we never cut and shut a working standard one, which is the way it was desinged for, What i'm saying is go out and spend the coin on a aftermarket one which has been proven to work, trust me, saves you in the long run!!
You said it yourself "as long as even flow is obtained to each port", clearly coming from the middle on this specific plenum was the way the flow was directed and made!!