For your sky textures, apply your 6 textures to an inside out cube in your modeling program and align them correctly. Then import the textures into UnrealEd and set all their Texture > UClampMode & VClampMode to TC_Clamp, so they will always clamp to the edges of the cube. Import your cube mesh, put it in the skybox & set it to unlit.
You may still find that on low detail settings, there may still be lines around the edges. This can be fixed when you create the DDS files, but I don't have time to find the link right now on how to do that - I wouldn't bother anyway, personally.
You may still find that on low detail settings, there may still be lines around the edges. This can be fixed when you create the DDS files, but I don't have time to find the link right now on how to do that - I wouldn't bother anyway, personally.