Probably not necessary unless you're running a tremendous amount of horsepower. The frame isn't really weak if its in good condition. The metal is thicker and there is more of it than many more recent cars and trucks in similar weight class.
People have done it, and one solution I though interesting was to use the sides of a spare frame to provide the fourth side.
i will have around 350 hp. in the process of installing the rear suspension I removed the rear crossmember. I know this adds strength. but with it out you can twist the frame by hand and it is in good shape. I will replace the rear crossmember with a piece of tubing and fab up a hidden reciever hitch.
