We present remedies to two fundamental difficulties facing the applicability of the traditional FFT-based beam propagation method (FFT-BPM) when investigating the propagation and transmission of transverse magnetic (TM) optical beams in subwavelength step-index waveguiding structures. To the best of our knowledge, the FFT-BPM is introduced for the first time to assess the plasmonic-dielectric waveguide interconnects. At the junction plane, we modified the FFT-BPM algorithm by including a combined spatial-spectral reflection operator formalism to calculate the reflected field. As a test, we calculated the optical power transmission efficiency between plasmonic and dielectric waveguide interconnect. A comparison between our results, and those obtained by full-modal matching using finite-difference frequency-domain (FDFD), reveals good agreement. Such interconnecting structure is crucial in many applications as biosensors, optical near-field probes, and interfacing elements involving high-contrast refractive index materials. We believe that rehabilitating the classical FFT-BPM to handle nanoscale waveguiding structures, which include metal-dielectric interfaces, will be of prime importance in the development, analysis and assessment of nano-photonics devices.