DONT LET THEM KNOW YOU'RE READY To ORDER!!!!! And don't act like a GM fanboy... make it known that you're in the market for a new truck and are considering the new Dodge and Fords as well (and insert a corny joke about Toyota like "i was considering them but im a fan of being able to stop")
Walk in there and say you looking for a truck with [insert your top 3 options here]. Then start "adding" options as you're looking at truck and the board in the showroom that shows you all of the available options. THEN when they offer to build/order one for you you'll have a bit more leverage to discuss price.
Once you let them know that you're there and WANT to order a truck, your price bargaining powers have been completely diminished.
Good luck

Edit: and if you can... get a price and then get the salesman name/phone number and ask him what his hours are. Tell him you're going to test drive some of the other trucks and work out prices and IF you decide on the GM you'll give him a call to get the order going. He may do more to keep you there and order it then and there... and if not, no big deal... just go to lunch or dinner and go back there in the evening to place the order.