figured I would just add to this post. I just finished my sleeve job.....
I also used 4x6 1/4 wall. I tried to find something less thick for weight reasons, but all I could find was 4" x 1/8" wall. It would've worked out pretty nicely, but I wanted it tight up against the frame as I plan on mounting all kids of neat stuff on it. So I followed the crowd. It's expensive shit, but you only need 5' of it rather than 10, so it will be cheaper than most I think, I paid 11/ft plus a cut fee. ~65 all together just for your information. I might have gotten it cheap, I might have paid too much, who really cares, that's just what I paid so you can base your prices off of it.
I just used two straight cut pieces, rather than 3 pieces like most. That little bend in the frame right behind the crossmember is not worth the effort of cutting and welding more.
The part that I figured I would post about is at the pinch seam in front of the crossmember. I cut it off completely, and welded it back up. It gradually gets wider from in front of the crossmember to the control arm bracket. so without the pinch seam, I just slit the piece of tubing down partially, not all the way off, leaving it as one piece. then I c-clamped it all on tightly, cut a sliver and welded it in place, then welded the whole thing on.
While I was at it, I welded up all the frame rail seams before I put the metal on. that's about it...figured I would add more info for others. _nicko_
here are some pics.
You can see the extra welded in piece there a little. I was too lazy to make it all smooth and junk, didn't think it was that important.