FWIW, I am planning on doing this for my XJ (I think the OP should not do it, I want to be one of the first :laugh

. I just need to finish shopping for a house so I have a garage or yard to work in where I can leave my "mess" out for a week or 2 while I work.
I measured mine and I am pretty sure the stock XJ unibody rails are 3.5x3.5". I planned on building a frame out of 3.5x3.5x1/4" (or maybe x3/16"). That measurement was made directly below the passenger seat floor pans since my floors are completely cut out right now. The unibody does widen out to maybe 3x6 where the front control arm mounts are I believe, however that extra width may have been for the UCA bracket or something. I didn't really look into it TOO much. I drew out my plans based on full 3.5" box (using long arms for the front suspension).
This project is kind-of hinging on the fact that I get my floor pans replaced and deem the Jeep to be worth throwing more money into. I think it will be, but I will have to wait and find out for sure