Longtubes, generally, will yield a more-broad torque curve.
One of the Mustang rags did an article on this very topic and gave torque curves for all of the different types of headers. The most significant item of interest was that the torque difference between equal length and regular shorties wasn't large at all, maybe 6-7 lb-ft if I remember correctly. Long tubes seemed to dominate, and the size of the primary tubes had *some* effect on the peak torque RPM. This is all from memory and experience with resonance tuning, so if I'm wrong....have a nice day.
