Why no body lift with diesel Chevy/GMC?

[Bulldog]

I've seen other older diesel Chevy trucks with 2" and 3" lift kits, so it must work. ORD sells a 1" lift which they said will work. I just bought their 1" zero rate add-a-leafs. This will give me 1" of lift and allow me to move the front axle forward and the rear axle back. If I don't get enough clearance with 6.5" of lift and some fender trimming I'll order the 1" body lift.