Interesting idea. I'v been entertaining the idea to do something similar but am trying to think of a way to work around my Anti-rock. Are you planning to leave the ends of the tube crossmember in the frame and just basically replace the section in the middle with a rectangular tube?
Also I'm 3hrs away from my Jeep or else I'd take a look but IIRC the crossmember up front is the only structural part forward of the engine I think? you'll need to brace the frame or weld something in temporarily before cutting out the tube or the frame rails might shift. I could be wrong, like I said, I don't remember for sure how far back the next structural piece is on the frame.