Albatross,
Here's how I started:
1. Basic levels/color correction. You did a better job than I
2. I used Filters-Distort-Lens Correction. Use the little thing that looks like a level to straighten the picture. Find something that is supposed to be true vertical - I used the edge of the window. Press OK and Photoshop will correct the perspective and angle. Magic!
3. Ever heard of a Rod Stewart song called "Every Picture Tells a Story"? No matter

I keep that in mind when looking over something that needs help. What story is this trying to tell? It's about the prom. We've got four teenagers all dressed up and I'd bet good money that one of those young men borrowed his dad's car (probably a 55 Ford) for the big night. The picture was taken by your grandmother with her Brownie Hawkeye. It is not the work of Ansel Adams and you're not desecrating it to do some cropping

. I took out everything that distracted from the story. I'd consider changing the background as well because it's distracting as well.
Cheers,
MJ