Spectrally multiplexed, high brightness, beam combining for power scaling and WDM communications applications has advantages over coherent combining, and is suitable for use with high gain bandwidth lasers. A diffraction grating forces oscillation at distinct wavelengths in a laser array in the external resonator configuration proposed by T.Y. Fan, et al [Opt. Lett.25 , 405-407 (2000)], producing a multi-wavelength output of overlapping beams. A unified diffraction integral-based treatment is used to formulate a design principle by which to jointly optimize most significant measures of performance, such as efficiency, bandwidth, and beam quality. The sensitivity to errors in alignment and positioning is also characterized. The transform lens aberrations are shown to ultimately determine the maximum array size, and the design criterion is applied to test and propose improvements in the lens. Initial comparisons show agreement within estimated experimental uncertainty margins with MIT/LL proof-of-principle measurements on an Yb-doped fiber laser array. To illustrate, we foresee a feasible rectilinear 100 - emitter array system using a multi-element aspheric lens, with <5%array-center-to-end efficiency drop, yielding beam quality M2 ? 1.2 in the transverse (array) direction, and close to unity in the lateral direction. Application to the efficient coupling of a laser array to a multimode fiber core is discussed as fmal example.