You wouldn't quite be allowed to fly a 747 without a type rating. However, you could act as SIC / co-pilot (assuming you meet other requirements such as IR and CPL in the case of being paid) of a two pilot turbojet including > 12500 lbs with no formality for GA operations (including a 747).
You may also need a Second In command type rating for operations outside of US airspace to act as SIC in a 2 pilot aeroplane.
You would need a high altitude endorsement if your turbojet is certified for flight above FL250 and high performance endorsement for whatever operation......