By David Shepardson, Allison Lampert and Tim Hepher (Reuters) -Spirit AeroSystems said on Tuesday it would receive up to $350 million in advance payments from its largest customer Boeing (NYSE:), giving the struggling supplier a lifeline as it burns cash after four consecutive years of losses. The Wichita, Kansas-based aerostructures giant is also on the verge of an agreement for a smaller amount of fresh funding and revised payment terms