I am trying to use Mosaic to New Raster in ArcMap 10.6 to mosaic 64 different DEM's from throughout WA state (the raster files were all collected from this website Washington 10-meter DEMS and are not contiguous). All of the original DEM's have the spatial reference of either NAD_1927_UTM_ZONE_10N or NAD_1927_UTM_ZONE_11N, are 32-BIT-FLOAT, have 1 band, and have a cellsize of 10,10.
I want to create a mosaic raster from these separate DEMs that uses the WGS 1984 spatial reference, but maintains all of the other original conditions. My assumption has been that I need to specify the cellsize in the Mosaic to New Raster tool to preserve the 10,10 cell specs in the original raster files (cellsize=10). However, each time I do so, I get an error report that my spatial references must be identical. I get this error even when I mosaic together only 10N or 11N raster files (assuming that's causing the "spatial reference" identity issue). The only time I get a successful mosaic with the WGS 1984 reference is when I DO NOT specify the cell-size (keeping all other conditions the same).
Can I trust that my data are reliable when the output mosaic does not have the 10,10 cellsize? What else can I try to get reliable data? I want to use the mosaic DEM to determine elevation, slope, and aspect conditions associated with plant distributions.