The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) and the Hinode/ EUV Imaging Spectrometer (EIS) observed AR 11726 on 2013 April 23. We present intensity images in numerous atomic lines constructed from these observations. These lines are formed over a wide range of temperatures, and we use their relative intensities to constrain a parameterization of nanoflare heating. We construct a 3D model of the magnetic field in this active region by extrapolating the surface magnetic field into the corona and using SDO/AIA images of coronal loops to ensure that extrapolated magnetic field lines co-align with observed coronal loops. We trace 2848 magnetic field lines within the volume of this active region and model how they fill with hot plasma in response to nanoflare heating. We perform a parameter study to determine how the frequency and energy released in nanoflares scale with magnetic field strength and loop length. From our 3D model, we construct synthetic images of the lines observed by EUNIS and EIS and constrain the parameter study by minimizing the difference between the synthetic and observed images.