By Svea Herbst-Bayliss and Lauren Hirsch BOSTON/NEW YORK (Reuters) - Billionaire investor William Ackman's success at Mondelez International Inc (MDLZ.O) may depend largely on a calculation that nimbler rival Kraft Heinz Co (KHC.O) will soon snap up the maker of Cadbury chocolate and Oreo cookies, sources familiar with Ackman's approach said. Ackman's $20 billion hedge fund Pershing Square Capital Management disclosed late Wednesday it spent $5.5 billion for a 7.5 percent stake in snack company Mondelez, making this the firm's biggest-ever bet. It is the second time around for the activist investor and Mondelez. Pershing Square briefly owned the company in 2013 after its split from Kraft in 2012. Investors and analysts agree there are several routes to higher returns. Costs could be cut further or the board may want to find a new chief executive to replace Irene Rosenfeld, 62. But the most likely path could be a scenario where Kraft Heinz, created earlier this year, reunites with... More