Your average car sales person is only trained on how to sell you what is on the lot. The entire system is geared towards that. As soon as you want a custom order from the factory, you should be asking to work with the sales manager who is generally the only person who can enter orders into GM workbench and understands the allocation process. Even then you probably won't get very definitive answers from them. From GM to the dealers they really really want you to just buy something on the lot. The whole system is setup to dissuade you from putting in your own order. You have to know this going in that it will be an uphill battle. It is called the "Goat Rodeo" for a reason.