Ignore the mounting holes on the radiator brackets. Lay a paint stick on the core support, set the radiator on it, and mark the spots for the new mounting holes on the brackets. Drill or file as appropriate.
The mounting holes on the Champion radiators are generic, and not specific to application, so if that's the only "problem" don't worry about it. If the radiator is too tall even when its sitting on the paint stick, then send it back.
