Boeing B377

Greetings. I'm planning on flying the B377 in the Around The World Race but have encountered some oddities. Others who use this plane can fly it at any speed below the max 305KIAS without problem. At cruise alititude of 20,500 I get an overspeed warning at 240KIAS. I reduced cargo and fuel to 108,000lbs and no change. It was suggested that I swap out the ASI gauge for one that doesn't have an overspeed needle which I did, also with no change. Cowls closed. Anyone fly this plane and have an idea on what's up? Is there a particular version that flies faster? I love flying this plane and want to keep it in the race. Any suggestions welcome. ThanksMiller