The grating prism (grism), slitless spectrometer aboard the Wide Field Infrared Survey Telescope enables a survey of emission-line galaxies. To facilitate its opto-mechanical alignment, a six-degree-of-freedom element was fabricated using alignment fiducials and integral flats and used to measure a wavefront by using an infrared interferometer placed at various field points over a 20 x 14 deg field of view in the grism coordinate frame. Modeling identified E2 to be the most sensitive element to the grism wavefront error and was used to efficiently align the system. The merit function regression method for a wide field of view was further used to verify the higher efficiency and accuracy of the proposed alignment technique compared with the conventional sensitivity table method. (C) 2019 Optical Society of America