Hybrid epitaxial boron nitride films combining centrosymmetric hexagonal (hBN) and non-centrosymmetric rhombohedral (rBN) polytypes offer a unique platform for engineering second-order nonlinear optical response in the ultraviolet. We investigate how the spectral profile of second-harmonic generation in such heterostructures can be shaped through the combined action of stacking-induced optical interference and the intrinsic spectral dispersion of the rBN nonlinear susceptibility. First-principles calculations of the wavelength-dependent χ⁽²⁾ of rBN are incorporated into transfer-matrix simulations of hBN/rBN stacks, revealing two distinct regimes of SHG: a localized-source regime enabling ultraviolet-enhanced excitation spectra, and an extended nonlinear-slab regime lacking any specific UV preference. Control simulations assuming dispersionless χ⁽²⁾ demonstrate that observed non-monotonic spectral features require the interplay of stacking geometry and susceptibility dispersion, and cannot be reproduced by geometry alone. SHG excitation spectra measured on polycrystalline MOVPE-grown BN films exhibit characteristic qualitative trends consistent with these regimes. These results identify mixed-polytype epitaxial BN as a promising route toward compact ultraviolet nonlinear optical elements, with polytype engineering emerging as an effective tool for tailoring SHG excitation spectra.