The dyno testing I've done with a stock type six pack compare to a single plane intake and 1050 CFM Dominator carb was that the six pack made 8 HP more at its peak RPM and similar torque.
All the stock type six pack set ups I know of make peak HP around 5500 RPM and peak torque around 4500 RPM, the bigger single carbs on a decent single plane intake make peak HP and torque a lot higher nearer 7000 RPM
I had a SS Weiand two piece 440 six pack intake I never tried or used