Re: A venerable workhorse
The B52 has 8 engines in 4 pylons. The US Air Force is not giving up on the BUFF anytime soon, and an airframe could easily re-purposed for engine testing, even if it meant taking off 2 engines and leave an entire pylon for the engine testbed purpose.
Fully unloaded, I doubt the B52 would face the same problems the 747 can already counter, such as the heavy yaw effect due to the uneven thrust. In fact, the B52 can even LAND way off the center line, since it has "crab landing gear", as in, the landing gear can also be twisted to make a heavy rudder landing, designed primarily for heavy crosswinds situation. The bomber is so reliable and the model so old, that in fact, the children of the first pilots already qualified and joined the Air Force and flew the bomber themselves.
Even so, nothing prevents them from building a whole new fresh 747 for the sole purpose of engine testing, even if the model has no longer any commercial application.