You can always try to leave it at the default resolution. Then move your seat back using the down arrow key and hit F10 to save this as your default setting. You must do this for all planes.
Or
You can buy a track IR and this will solve your problem of not being able to quickly see some of those lower gauges.
Or
You can somehow, add black bars that run up/down on the left and right side of your monitor and run Aces high in the original 4:3 ratio resolution, but you must have this option in your video drivers. unfortuneately, I do not know how to implement this option. I've been told it has been done before. This way, you do not get anything cut off or stretched in the process.
Or
You do nothing and play with it in a stretched mode and have nothing being cut off.
Good luck.
BTW, I went with the TrackIR route.