i'll throw my .02
about the surge......IMO....it aint the exhaust duration.
it sounds like the dyno load valve was reaching the end of its travel at the upper end of the pull.
this is often an issue with an SF dyno when you have a large cube/high torque motor and try and make pulls from a relatively low starting rpm, and try to make pulls that extend over 2500rpm.
the absorber isnt that efficient at lower rpms, so you need to run a lot of water through it to hold the load.
as the rpm rises, the efficiency rises very quickly, and the load valve has to open way up to get rid of that extra water, and you run out of travel on the valve at a reasonably low rpm.
if you start a few hundred rpm higher(where the absorber is more efficient), you can restrict the inlet water flow, which will generally allow you to run to a noticably higher rpm before the servo valve gets near the end of its travel.
if it wasnt a load control issue, then my vote would be for some sort of valvetrain dynamic issue(looking at how steeply the TQ curve falls off the last few hundred rpm).
in any case....those are quite good results from that basic combo.
