Belgian financial investor RHJ International offered 275 million euros ($387.6 million) for a 50.1 percent stake in General Motors' Opel business in a last-ditch effort to beat out rival suitors.

RHJ's non-binding offer document, entitled Project Beam and obtained by Reuters on Friday, foresees production cutbacks and pay cuts for staff at the German carmaker which GM is selling amid a radical slimming program to restore profitability.

With the plan, RHJ hopes to outmaneuver Canadian auto parts group Magna International Inc, which has looked the frontrunner for Opel since it signed a memorandum of understanding with GM to acquire the Ruesselsheim-based group nearly two months ago.

German Chancellor Angela Merkel voiced support for the Russian-backed Magna plan at a news conference with Russian President Dmitry Medvedev near Munich on Thursday, but she also said the Canadian group had to do its bit to seal a deal.

The RHJ plan requires 3.8 billion euros in government guarantees, less than the 4.5 billion Magna is demanding.

According to the document, the group would contribute 175 million euros at the closing of a deal for Opel and an additional 100 million at the end of 2012, good for a narrow majority stake of 50.1 percent.

GM would keep a 39.9 percent stake in Opel and employees would hold 10 percent in exchange for agreeing to contribute between 250 million euros and 300 million in savings per year.

RHJ would reduce Opel headcount by about 9,900, shutting a plant in Antwerp, Belgium in March 2010 and downsizing the carmaker's four German plants, but keeping all of them open.

RHJ projects net sales would rise from 16.9 billion euros in 2009 to 23.6 billion by 2014, with EBITDA pushing up to 2.5 billion by the same date.

The Magna offer also envisions about 10,000 job cuts across Europe and no outright plant closures in Germany. In addition to RHJ and Magna, China's Beijing Automotive (BAIC) is also pursuing Opel, although sources have told Reuters its chances of success are not as strong as the other two bidders.

(Writing by Noah Barkin; Editing by David Holmes)