Boeing and GE used a modified 747 as a flying testbed for the GE-90 engine used on the 777. The engine was mounted on the inboard pylon on the left wing. It definitely looked unusual with the three normal 747 engines that were dwarfed by the -90. According to what I was told, two of the -90 engines could have replaced the four smaller engines, but Boeing had already decided on the 777 platform to eventually replace the 747. In order to convert the 747 to a two-engine design would have required redesigning the original wing to position the engine pylon between the two engines.