An effective engineering method for efficient coupling between a tightly focused sub-5-nm-gap plasmonic crystal cavity and an integrated waveguide is proposed. A three-dimensionally tapered 2-nm-gap plasmonic crystal cavity exhibiting a mode volume of 6x10(-6) (λ/n)(3) can efficiently couple to an integrated waveguide with over 90% efficiency by optimally selecting the number of air-holes in the plasmonic crystal. The field intensity is accordingly maximized, and its enhancement reaches to 205,000 for the light incident from the waveguide. Here, the optimal coupling is achieved by matching the radiation rate of the cavity mode to its absorption rate. In addition, the strongly enhanced field intensity boosts the spontaneous emission rate of the dipole emitter embedded in the cavity. The maximum radiative Purcell enhancement is calculated to be 362,000 where the quantum efficiency exceeds 50%.