I don't think it would meet the engine failure on take-off requirement.
-90 and similar are rated to allow a 777 to continue the take-off safely if it loses one past V1, but a loaded 747 struggling airborne on one GE
-90 would surely be quite interesting.
It would need a serious pylon re-design aswell. The one on the testbed 747 has insufficient ground clearance for (a) ordinary pilots and (b) gusty conditions. One on each side would mean landings would have to be consistently very close to perfectly level (in roll).
Since we're not all test pilots, and can't guarantee to keep it at zero bank on windy days, they'd need skids under the pods!!
QantasA332 - Agreed, the wing was designed for the existing engine arrangement from the bending moment standpoint. And other considerations aswell. In contrast, the A330 engines are hung at the same point as the A340's inners I believe. But this was designed into it so they could use a common wing.
Regards - Musang