There is no stock OEM head that will flow the same as a good new after market bolt on replacement head, I've had many stock heads flowed on different benches and then had Eddy and 440 Source heads out of the box flowed on the same benches and the new head out flow all of them, including the stock M.W. "286" 1962/63 castings shruggy
I had a set of 906 heads ported and polished and upgraded on the valve sizes to 2.14 and 1.81, they flowed 266 CFM at .700 lift. I bought a set of Eddy RPM that Modern Cylinder heads CNC ported and they flowed 310 CFM on the same bench work
My old pump gas Duster ran a best ET of 10.69 at 124.6 MPH with the iron heads and just changing the heads(both sets had 84.0 CC combustion chambers) the car ran 10.49 ET at 127.5 MPH work
I ended up replacing the RPM heads and six pack with a set of Indy SR with M.W. ports and a 400-3 intake with a Holley 1050 CFM Dominator, that let the car run a best of 9.993 ET at 134.* MPH through the complete exhaust system with the air cleaner on boogie
More air and fuel makes more power work
My message is to buy the best heads you can the first time up

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)