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

Part of the trick (in my mind) is figuring out what is bent. It could have one leg bent up or down and the other 3 are fine. Or the whole think could be twisted. Both of those scenarios would give the same results as what I have already done.

Thought maybe the motor mounts could be used to figure out a plane and then measure from that plane. But not sure the motor mounts are even supposed to be in a plane that is parallel to the frame horns.

I found posts by @Jim Lusk where he straightened some, but not sure how other that at least once he cut the thing apart.