Another Solution could be to create polygons the size of the texture. If you make a box 512 x 512 with a 256 texture, change it's segments from 1 to 2 will -give a poly face for the texture. Using uvmap - box or uvmap-faces will give you perfect texturing as the face is exactly the size of the texture.

Edit NOTE - This is for exporting from max as fbx - in which case you'll have to export the level piece by piece and reconstruct it in WED.