What is a "Pod-Pak"?
A Pod-Pak is a aerodynamically shaped enclosure to transport an extra engine under the wing. 

From 'Flying Magazine', Nov 1959:


  Spare engine in “Pod Pak” is the airlines' modern method of
  transporting extra engines to maintenance points. Nine major airlines
  flying Boeing 707s have ordered Pod-Paks. This spare suspended below
  the wing of a Qantas Airline's 707 shows the Pod Pak suspended from
  the airplane's front wing spar by means of two fittings and to the
  landing geard support rib by four bolts. Fairings streamline ends of
  the engine and normal engine cowls enclose the sides. No structural
  changes are required in the airplane's wing and, when not needed,
  Pod-Pak can be removed completely.


The exhaust pipes of the Pratt &amp; Whitney JT3C turbojet engine on the left of the picture are an attempt to reduce the noise of the engine. They should aid in the mixing of the core exhaust stream and the freestream air which is quite a noisy process. 

The successor to the JT3C, the JT3D, was a turbofan. In a turbofan there is a layer of bypass air between the core exhaust stream and the freestream which reduces the noise and increases the efficiency. Later B707s fitted with the JT3D engines did have not have the 20 pipes exhaust. Most of the aircraft which were originally fitted with the JT3C where later retrofitted with the JT3D.