It appears the mph is very low for the et which is telling me your gear isn't matched to your revs.

You must be nosing over on top? What rpm do you shift and what rpm at the traps?

Two ways to skin the cat:
-get the motor to rev higher (per suggestions above, i.e. better flowing heads, intake, etc)
-use a taller gear (numerically lower) to make better use of the torque down low...

Sounds like the cam is a high winder so either use a shorter cam and shorter gears or better heads

Two cents...

-Chris