GMC - Sierra :: 2004 - Blowing White Smoke From Tail Pipe And Coolant Consumption

I have a 2004 GMC Sierra 2500 Duramax diesel pickup. It has been blowing white smoke from the tail pipe and drinking coolant, 10 - 15 gallons since February. There are no obvious leaks of coolant under the truck. However, within the last 3 weeks it has on 4 occasions spewed coolant from the surge tank from the overflow vent. We have narrowed down the problem to either a headgasket leak or an injector leak.

We had it in to the dealership (big mistake) and they found nothing wrong. They think we either need to wait for something catastrophic to happen or do the head gasket and replace the heads. There is a technical service bulletin about the head gasket. There is anecdotal info from an article written by a master tech from GM about the injectors. We also know there is something about GM paying for the injectors and injector sleeves to be replaced if there is an issue like we have.

All the dealership is saying they did was drive around the truck and saw no coolant spewing forth and no smoke. They did see a lowering of coolant level in the surge tank. They also did another pressure test on the coolant system and found nothing. Outside temps and weather conditions have no effect on this situation unfortunately nor does the length of time it's driven. How much should a head gasket/head job cost? How much time should it take? Does it require removal of the cab? How much should an injector job cost? How much time should that take?