The crew/ext cabs are a lot of work to do. I've been working through mine for a year now and this is where im at. I'm extending it 16 and a half inches and in just that length, the variations in width of the roof from front to back are very noticable. We decided to take all the panels apart and take it one piece at a time, strengthening every joint.
In the pics you can see the inner roof going in and some of the other stuff going on.
If you want a good look at a killer crew cab, check out dino63's
http://67-72chevytrucks.com/vboard/s...d.php?t=327006