Fifth pod between #2 and the fuselage was the main way to ferry a spare engine. For a time my company was overhauling engines for a Chinese operator. We fifth podded regularly between Sydney, Beijing and back.
On one occasion we carried an engine for a Flying Tiger 747 100 which was a slightly smaller diameter. It was a Sunday afternoon and we were transiting Melbourne. When we moved the engine dolly in it couldn't raise enough for the early series engine so we let the aircraft tires down. Was a good idea until we discovered that there was no nitrogen available until Monday
As far as similarity of the electrical system to the B707, the 747 was so similar for most systems including electrical that when we converted to the 747 from the 707 we didn't do a type course, only a differences course.I seem to remember that the VC10 electrics were a bit similar but quite a few differences.
In an earlier life I was licensed Elec and Instruments of the B707. From what I remember of the VC10, it wasn't that close to the B707