Was a six-engine 747 ever seriously considered by Boeing?
The "fifth engine mount" option on the 747 is not designed to handle a running engine. It was an option used only by Qantas as a means of ferrying spare engines to remote locations, where flying a plane for a long distance to a maintenance facility on three engines was not possible. Only four of the Qantas fleet of 747s (totalling more than 60 aircraft) had this fifth engine option. 

The mount for the fifth engine was not designed to transmit any thrust the engine would have delivered if it was running, and it fact the engine is partially disassembled by removing the fan to reduce drag (which reduced the loads on the 5th engine mount, as well as drag on the plane as a whole).

Some pictures and videos here: https://www.flightradar24.com/blog/how-qantas-ferried-an-engine-on-the-wing-of-a-747/

To convert this into "a 5 or 6 engined plane" would require a lot more than just "a few pipes and wires". The wing structure would have to be redesigned to handle an extra 50,000 pounds (or more) thrust from each extra engine, plus the extra weight of a proper pylon and nacelle structure. Considering the aerodynamic wing flutter problems with the initial 747 design, sticking another two engines on the wing would most likely have required a complete redesign of the wing.