The ypipe is big enough to flow more than enough for a N/A 5.3 or 6.0, just make sure the rest of the exhaust is free flowing, mine's 3" from the headers back. I still need to go re-dyno it though.

The flanges are plenty thick but i'm not sure the exact thickness. As far as emissions go you would have to get the codes tuned out through the PCM via a tune. The extra o2's after the cats get removed along with the egr system, they can all be tuned out of the computer. You'd have to find a place that is a little relaxed on the visual part of the inspection.