These will never flow like headers but they do help a little.
Mopar Muscle magazine did a test on a crate 360 many years ago, comparing the HP and torque numbers using several different manifolds, then headers. The 340 manifolds added a surprisingly small power increase over the basic 318 manifolds. Headers added far more. The 340 manifolds added something under 10 HP, a number you can hardly feel.
I did my own research on this subject many years ago and figured that while every little bit helps, it becomes pointless if you are spending a lot of time and effort to get manifolds to fit because the gains are so small.
