The spectral turbulence transport model from part I of these two articles is compared with experiments of the late stages of turbulent mixing due to Rayleigh-Taylor instability, when the dynamics become approximately self-similar in response to a constant body force, We also examine the spectral dynamics of the flow immediately following a rapid transient in the body force. Generic forms for anisotropic spectral structure are described and then used as a basis for deriving spectrally integrated moment equations that can be more easily incorporated into computer codes for scientific and engineering analyses. We describe comparisons with experiments of this spectral formulation and indicate directions for refinement, analogous to those that are required for constant density turbulence transport.