How do you straighten a k-frame off the car? Is it worth it?

As strong as they are, without access to a frame rack or some other "strong" device, I don't know how you would. Getting dimensions for the 4 mounting holes is not tough, and they have to be in the same plane If it's much more than that I don't you'll be successful. If you had to pay a guy to put it on a frame rack, it's likely a goner