I don't know what you're doing wrong when it comes to saving your photo in PS, but here's my crop-suggestion:
When it comes to pan shots, I learned a thing on a media course once upon a long time ago:
Always keep a little more air in front of the vehicle than at the rear.
It makes sense for the brain when it comes to movement in a still photo or a moving image.
If you just want to show the P-51 as it is - a really nice picture showing the plane as it
is - crop it so that it is centered.
Like this rough version:
If you want to keep a bit of impression of the forward movement of the plane, leave a little more space from front end of plane to edge of picture.
Like this rough version:
I've rotated the image a little (0.85 degrees) so that the runway that forms a line in the photo, gets levelled out.
Then I did Auto Levels and finished off with Unsharp Mask - I usually use this configuration for the Unsharp Mask.
This way the UM don't get too harsh - i.e. it doesn't show that you've used it, as some PS users mistakenly tend to use a config of the UM that's too harsh in my opinion:
Amount: 60 %
Radius: 2 pixels
Threshold: 10 levels.
I'd prefer to crop it like this, but it's also a matter of personal preferences, and it also depends on what you're going to use the photo for: