Fiat has more than a 50 percent chance of pulling off its bid for Opel, Italian daily La Stampa on Thursday cited Sergio Marchionne as saying to members of the Agnelli family.
The Agnellis founded Fiat more than 100 years ago and still hold a controlling stake in it. They also own La Stampa.
GM is seeking an investor for Opel and its other European brands as it hurtles toward a June 1, U.S. government deadline to complete restructuring talks with stakeholders.
Three bidders submitted proposals to the German government on Wednesday.
Along with Fiat, Canadian-Austrian car parts group Magna International
Fiat would appear to be in the hot seat as the most credible potential partner for Opel, said IHS Global Insight analyst Margaret-Anne Orgill.
However, German magazine Spiegel Online said Fiat was at the bottom of a list of three contenders, citing what it described as an internal GM ranking list of the bids.
Citing unidentified GM insiders, Der Spiegel magazine said GM had reservations about Fiat because the Italian industrial group was a partner of its U.S. rival, Chrysler LLC
The German magazine said that Magna is at the top of GM's list, ahead of RHJ International.
Fiat wants to merge its car business with Opel in an ambitious plan to create the world's second biggest group after Toyota Motor Co <7203.T>. Its bid includes GM's British brand, Vauxhall, but not Saab, which is being sold separately.
While Fiat could raise synergies from existing shared platforms and powertrain technology with GM, it would have to massively cut capacity and head count, IHS's Orgill said.
Fiat's Marchionne told German magazine Spiegel Online his plan would require job cuts at all four German factories as well as most sites throughout Europe.
We need to cut production capacity at the new joint company comprising Fiat and the European GM units by 20 percent, Marchionne told the magazine in an interview.
GM will consult the German government before choosing the winning bid because Opel, which is based in Germany, will likely get billions of euros in financing from the government.
How far Berlin should go to prop up Opel, which traces its roots back to the 19th century, has become a topic of fierce debate ahead of a federal election in September.
Chancellor Angela Merkel and other top politicians are set to meet on Friday to review the proposals, sources close to the German government said.
And in case none of the bidders have a winning concept, unions have prepared an emergency rescue plan, German daily Frankfurter Allgemeine Zeitung said.
Opel's works council has invited the bidders to present their proposals at a meeting at Opel's headquarters in Ruesselsheim, Germany on Tuesday.
Also in the coming days, Fiat officials will meet union and Italian government representatives to discuss the car maker's plans, Italy's Industry Minister Claudio Scajola said.
(Additional reporting by Stephen Jewkes in Milan, Stefano Bernabei in Rome, Peter Starck in Frankfurt, Klaus-Peter Senger in Berlin, Dmitry Zhdannikov in Moscow; Editing by Dan Lalor and David Holmes)