Soulja Boy Says Evander Holyfield's In His Corner, Floyd Mayweather's Out

Soulja Boy's stepping up to the heavyweight ranks for his fight with Chris Brown -- he's got Evander Holyfield training him now, instead of Floyd Mayweather.

Soulja's promoter, Wack 100, tells us he reached out to the ex-champ and hooked them up on the phone. He says Soulja pitched himself to Evander ... convincing him he's serious about the boxing match with Chris.

We're told Soulja did a lot of talking about "knocking out" Chris, but Evander gave him a quick tip: train harder, talk less. He also assured the skinny singer he can get him into true knockout form. Of course, the nemesis of Evander's ears -- Mike Tyson -- is training Chris, but we're told his name didn't come up during the initial convo.

As for why Floyd is out ... Soulja felt like he was too busy with other interests. We're told there's no hard feelings, just business.