How to UV unwarp and keep the original face proportion? I have a simple cube and in edit mode , i stretched it in Z direction  so now the cube become very tall and the faces on top and botom are still perfect square. I setup the edge seams already so it will unwarp the box perfectly. The proportion of uv will be correct if i use square image (texture). If my texture doesnt' have 1:1 aspect ratio, the uv will try to adjust itself to the image aspect ratio. How to ignore image/texture aspect ratio ?
If your texture is 800x600 do some simple math: 600/800=0.75, so scale the whole UV by 0.75 on X axis (A to select all UV, S X 0.75 Enter).