Slight bend in frame...

[sick472]

A Crescent wrench should work very well. Start towards the front (or back) and work it a little at a time, switching front to back as you move it a bit each time, working closer to the worst of the bend as you go. Keep doing this until it's straight. If you start at the worst part with a hefty bend, it WILL go back, but this bend will STRETCH the metal too much on either side of the initial bend. This will leave you with a wavy repair. Not real critical, but you can get a better result with some patience and just working it back a little at a time and moving from front to back as you dial it in.