I installed my DC sports a little while back. It was kind of a pain getting in the tight spaces, but I didn't rush it and it went well. I used PBBlaster on the nuts and let it soak in, not sure if it helped much though. The nuts were in good shape, and they came off fairly easily. Just need to make sure you have extensions and flex/u-joints.
6 hours sounds about right. Although, I would think a shop can do it in 4 hours with a lift and all their tools. I think it took around 8-10 hours total in my garage, on jack stands, taking my time and wasting some time trying to get the O2 sensors off the manifolds. Only one made it off, so I ended up ordering a new pair instead. That's another thing you might want to do to make it go quicker/easier. Definitely a great DIY job if you don't mind taking a lot of stuff apart. Oh yeah, I had to remove the rear (IIRC) motor mount bolt and lift the engine a tad to get the header in - it needed a little extra room to make it past the rear crossmember.
If you plan on doing it let me know and I could help you out, or provide moral support :tongue: