Match Race = 1968 Road Runner '383' vs. 1964 Belvedere '426-S' Street Wedge

Real world testing has shown that even the upswept HP manifolds (which was also rear exit) are just as restrictive as plain log manifolds. Log with rear exit will be just as restrictive as center exit, if not more.

So basically both are stuck with restrictive exhaust. The fancy upswept HP manifolds on the RR won't factor for much. IMO it comes down to the 426 having +43 inches and a more favorable torque curve to deal with the B body weight. Give them both enough bite and the 426 will jump ahead and stay ahead every time.