Head gaskets should be about 30 hours if your taking your time and it the first time your doing them. If you have a good background and all the right tools, you can cut that down some. Longest part is gonna be touring the heads down correctly. I tackle stuff by looking at it as a bunch of bolts, because that's all it is. Just a bunch of bolts that go different places. If it were my truck, I would have the heads checked to make sure they are flat and not cracked but they should be fine. That dealer quote was probably close because they charge by the book. And figure 30 hours at $100 an hour. SEPS is a good shop that does not charge an arm and a leg. Used to use them at my old job all the time and a friend of mine is having them replace his injectors in his LB7. I would use them if I didn't have access to a shop full of tools and a lift and a oil field taught diesel mechanic and enjoy doing it myself.
Reason I say do studs is because they will not stretch and are reuseable. Stock bolts are not and can and will stretch. Not all that common on a Stock Duramax but if your gonna do it, you might as well do it right.
