More specifically, I need the GridMap itself to allow placement of (1 x 1 x 1) meshes on a (1 x 0.5 x 1) grid.
Changing the Spatial Scale of a 3D GridMap does something similar to what I want; but it scales down the Y value of the mesh blocks which causes a squashed appearance to the side of my meshes.
Sorry if this is confusing and thanks for reading!