Dan the long ram setutps were never really developed for HP, but rather torque. Keep in mind the factory hot rod they were standard equipment on (the letter series Chrysler 300) were over 4000 pound land barges. The numbers were pretty respectable for the time, and even today..the 375 HP version made 495 Ft Lbs of torque at 2800 RPM.

Here's an article on the manifolds theory with an interesting torque chart.

Ram Theory

From personal experience, I can also tell you that these manifolds do not work well when until they are fully warmed up as the fuel tends to fall out of suspension and puddle.