$$ Advice - 2015 Silverado LT 3500 SRW crewcab 4x4 w/ Diesel

I'm in the same boat. But getting a GMC 3500HD SLT Duramax. Start at NADA and build your truck exactly like you want to get dealers invoice. You should pay less than invoice if getting the right deal. You should end up paying 10 to 15 percent below msrp less any rebates.
If you are trading you will mud up the waters. If you can try to sell yourself. Also the dealer is more inclined to work off lower margin if it is a factory order. Don't let them push you to buy what you don't need or want just because it is on their lot.