Think about thrust !
The relative position of the thrust line (below, on, or above the fuselage centre-line) also has its effect. Also, remembering Vickers/BAC information handouts of the time, mountiing the engines aft makes for a "clean" wing.
However, the effect of engine weight far aft means that, to get the CofG right, the fuselage forward of the wing needs to be longer than the rear section (think DC8 in its later manifestations), whence a reduced tail moment arm, and probably a much larger fin (see the VC10).
This extra weight isn't necessarily compensated for by the ability to have a shorter undercarriage than when engines are mounted under the wing.
(I realise that this is a bit off the "stability beam", but it's worth remembering that apart from the VC10 and its Soviet "relation", rear-mounted engines are now confined to relatively small aircraft, by modern standards).