The large cross section of high BPR engines will probably force them up from under the wings.
There are known disadvantages to putting all your engine weight in the tail. GTF makes the matters only worse!
What does the GTF do with 13 and half tons of thrust? Mitsubishi MRJ 70 is supposed to have just 7 ton engines, and Bombardier C130 has 10 and half tons of thrust each engine!