Tubing diameter is larger than the 3.0, but thats not really the problem.
Everything bolted up, the only thing is the 4.8 has flanges that are larger in diameter.
So the only fabrication needed to fit it was to cut the flanges off the stock 3.0 axle back sections approx 2-3" back I think? Then they just welded the 3.0 flanges on the 4.8iS sections and fitted it like factory. I can take a pic underneath my X if needed
Basically reference this post on a 4.4 because the 3.0 and 4.4 share the same flanges on their exhaust and thats almost exactly what my shop had to do:
http://www.xoutpost.com/742961-post13.html