OK- i know you said smallblock but I think this may help.I had a 69 Superbee and was running a stock style 6-pack. Car was running 12.60's at the time and I had an M-1 single plane intake and a new Holley 750 vac. secondaries carb.Went to the track and made 2 passes with the sixpack.12.60 and 12.61. Switched the intake and carb to the 4 barrell and went 12.61 and 12.61. So in my application I saw no difference. When I installed the new engine it ran a best of 11.60 with the sixpack and didnt put a four barrell on that combo.