If I put your 320 cfm head on my flow bench, and flow it at .700" and 28", then bolt a stock RPM to it, depending on which port, the flow may drop to 270 cfm. Same head, same intake, but #1, #4, #6, or #7, and the flow may be 300 cfm. Bolt your 401 cfm on it, and it may or may not flow 320 cfm, but it will be close. Now take that same 320 cfm head and bolt the stock RPM on it, and pull the flow bench up to 34", and check flow, pull it up to 40" and check the flow. Did it reach 320? Probably, but just barely. Now put that 401 cfm intake on the same port, and pull the flow bench up to 34", and it may be flowing 330 cfm through the head. 40", it may be 340 cfm. The 28" is an industry standard, and it just measures how much something has improved, or is capable of being improved. Take this example. I have had some of these FE heads that just will not flow above .650" and goes turbulent. Put an intake manifold on that head, and it continues to flow to .750"----go figure. Another example: a head goes turbulent at .500", and I pull the flow bench up to 32", it is still turbulent. Pull it up to 40", and it does not clean up. That engine will be a dog. Another example: a head goes turbulent at .650", flow drops to 20 cfm at. .700", and drops even more at .750". The port had some work done but it was in the wrong place, and you can't fix it. The best answer for this question, is than on my 2018 EMC engine, the heads flowed 303 cfm upper/290 cfm lower/210 cfm exhaust, and the intake manifold flowed 338 cfm. Increased the intake manifold flow to 360 cfm, no difference. 375 Y, 561tq/595 hp. Joe-JDC