The engines slung under the wings help prevent fatigue issues with excessing wing bending with lift - the engine weight helps oppose it.
When I did my tanker ground school in 1962 it was at that time believed that the VC10 tanker was just around the corner. They even had built up instruction boards showing the fuel system etc.
The Valiant always flew with two 12,500 lb capacity underwing tanks, normally full at takeoff and transferred when the fuselage and wing fuel could accept it. Even then one would balance the fuel 66/33 % fuselage/wings. They Valiant was withdrawn in from service in 1965 owing to fatigue corrosion in the wing roots. Would the Valiant have operated without these tanks then the spars would have failed much earlier.
The VC10 tanker could well have been in service by 1965.