The Hummer headers are tubular, but are not long tube. I imagine they'd help a little with power, diesel apparently really respond to free flowing exhuast. We'll see i guess.
As far as muffler choice goes, from what i understand the big thing with diesels is soot. Because of this the mufflers to avoid are glass packs which will eventually clog with soot. A Flowmaster is a pretty free flowing muffler and should do fine behind a 6.2. I currently run a single 2.5" 50 series behind my 6.2 and it does fine.
My plan is dual 3" 40 series flowmasters and the Hummer headers. I've heard with larger dual exhuast the NA 6.2 sounds very similar to a big block.
Rene