The 427 is an FE family engine, which was used in NASCAR. The engine used in the gt40 was based in a NASCAR build of the engine (modified to handle right turns ;) ).
Back then, you could order performance parts from dealerships and whole engines. Could you walk in and order a gt40 engine? I have looked and have not found a part number. However, some race engines did not have part numbers because they were aftermarket parts.