There has been a recent renewal of interest in anomalously high energy transfer through otherwise opaque metal film geometries. One of the earliest such systems, that of surface plasmon cross coupling in a sinusoidally corrugated continuous metal film, was studied by Gruhlke et al. [Phys. Rev. Lett.56, 2838 (1986)]. We show that it is possible to use a nonperturbative method to accurately predict this coupling, and we use this method to analyze both intrinsic coupling in symmetric structures and grating-assisted cross coupling in asymmetric structures.