If it's rusted, there's a good chance it's rusted more than you can see. I have a 69 I'm restoring, I saw a little rust on the frame rails. Now that I've gotten everything out of the way I have found I need both frame rails, torque boxes and both shock towers.
I'm not trying to scare you, but just remember rust usually comes from the inside. So just check it out real well. You might be lucky and you could patch it with some heavy gauge metal. You can get a whole frame rail from CJ Pony Parts for less than $130.
Good Luck.