I've tested a single 6 pack Fram paper air cleaner versus two of them stack on top of each other together and then a single K&N and then two K&N and finally a six pack air cleaner base with no air cleaner and it made 5 HP more with no air cleaners, 7 HP more with two paper elements compared to one, same results with the K&N elements
My motor was much smaller, 511 C.I. with a 4.250 stroke and 4.375 bore with 9.25 to 1 comp ratio, it made the best HP, 612, at 5500 RPM and the best torque of 644 Ft. lbs. at 4500 RPM.
I flogged the heck out of that motor testing three different sets of carbs., ignition timing, valve lash and air cleaners that day
Once the motor was broken in and leveled out I saw 12 HP gain from worst to best parts tests