We apply a method for the construction of rotational-intermolecular vibrational states to the van der Waals molecule benzene-N-2. The method, as described in Felker [J. Chem. Phys. 114, 1233 (2001)] is based on the transformation of approximate rovibrational eigenfunctions from functions of coordinates referred to an Eckart body-fixed axis system to functions of coordinates referred to a more convenient body-fixed frame. This transformation allows the computation of approximate rovibrational eigenenergies, as well as other molecular properties, from one or a small set of J=0 eigenfunctions. By comparing rovibrational energies and matrix elements computed by this Eckart method with those computed variationally with a large basis, we show that the former yields excellent results for benzene-N-2. The results are particularly significant because of the presence of essentially free internal rotation in the species, a large-amplitude motion that complicates the definition of an Eckart frame.