Crop Canvas?

[ChameleonScales]

You can already do it this way :

1. pick the Rectangle selection tool
   
2. in the tool settings, check "fixed" and select "Aspect ratio" in the dropdown menu (or whichever you need)
   
3. input your values in the field just below
   
4. make your selection in the canvas
   
5. if you want to move your rectangle vertically or horizontally, you can pick the Move tool, switch to "Selection" mode in its tool settings, then use Ctrl while dragging the rectangle to constrain the movement along an axis
   
6. Once your rectangle fits your desired crop, go to Image > Fit canvas to selection. This will preserve all the pixels outside the canvas.