I'm totally amazed at those engine mountings.
Must I assume from the pic above that the top of a 747's bubble is strong enough to take the asymmetric thrust of an extra engine. or will there be some structural work inside?
Why ever would you want to test an engine like that when the empty 747 could manage perfectly well on three engines regardless of whatever's being tested?
Why wouldn't the front, top mounted engine muck about with the tailfin?