I swap from a set of mildly ported big valve(2.14,1.81) 906 to a set of MCH CNC ported Eddy 84 CC RPM heads(same CC as the 906) and my old pump gas Duster went from 10.69 at 124 MPH to 10.49 at 127 MPH with no other changes There was around 50 CFM flow differences between those heads also, 260 to 310 CFM
EDITED: Both heads had 84.Cc so the compression ratio stayed the same at exactly 9.25 to 1.
I change the crankshaft stroke the next winter to increase the compression ratio, went from 4.25 with the piston deck height at -.025 to a 4.300 stroke crank that made the pistons measure zero on the decks and that raise the compression ratio up to 10.29 to 1 which help the car run low 10.30s at around 127MPH also due to not having enough fuel volume in high gear back then that was with alow deck eddy six pack intake with 1970 440 six pack carbs set of by C&J engineering in Whitier, CA
I ended up swapping the heads again later to a set of Indy SR M.W. ports with 75.0 C.C. combustion chambers and a Indy 400-3 intake with a Holley #9375 non HP 1050 CFM Dominator and fixed the fuel delivery problem which help make the car run 10.00 at 134.+ MPH, one 9.993 at 134.89 run through the full exhaust with the air cleaner on running Oregon 91 octane non ethanol pump swill