I did, kind of... Here are some pics. Nevermind the welds in the third photo. It was with a flux core, before i brought over the real welder. I went to the junkyard and cut out the passenger side of the frame and cut it to fit for the drivers side. Why GM left tis section out, i have no idea, but if you are going to do a roll cage, i think this is a must.
Not completely boxed in, but i think it will help with flex.
Adrian