(For a Reuters live blog on U.S., UK and European stock markets, click LIVE/ or type LIVE/ in a news window) * Boeing rises on Southwest order for 737 MAX jets * Bank stocks fall after default by fund manager (Updates to close) NEW YORK, March 29 (Reuters) - The S&P 500 ended nearly flat on Monday, with bank shares falling amid warnings of potential losses from a hedge fund’s default on margin calls, while optimism over the economy limited the day’s declines. Shares of planemaker Boeing Co rose after the company reached a deal with U.S. budget carrier Southwest Airlines Co for a variant of the 737 MAX aircraft. Nomura and Credit Suisse are facing billions of dollars in losses after a U.S. hedge fund, named by sources as Archegos Capital, defaulted on margin calls, putting investors on edge about who else might have been caught out. Shares of big U.S. banks and even regional banks fell. “There’s still chatter as to whether or not, and which, American banks may be affected. That is a question that’s lurking. But so far the market has taken (the news) in stride essentially,” said Quincy Krosby, chief market strategist at Prudential Financial in Newark, New Jersey. Optimism about speedy vaccinations and record stimulus, which drove the Dow and the S&P 500 to record closing highs last week, helped keep a floor in the market along with upbeat estimates for upcoming earnings, she said. Unofficially, the Dow Jones Industrial Average rose 104.61 points, or 0.32%, to 33,177.49, the S&P 500 lost 3.25 points, or 0.08%, to 3,971.29 and the Nasdaq Composite dropped 79.08 points, or 0.6%, to 13,059.65. The Nasdaq was on track to post its first monthly decline in five months. Investors may also be adjusting their holdings for quarter-end “window dressing,” Krosby said. (Reporting by Caroline Valetkevitch in New York Additional reporting by Devik Jain and Medha Singh in Bengaluru Editing by Maju Samuel and Matthew Lewis)