"...in my mind, if you were to put a 747 into orbit, the internal air pressure and the outside vaccum would surely make it rip open? the space shuttle must be made of some strong stuff..."
Not really. When things were going slow for Boeing in the early 1970's, they proposed the 737 fuselage to NASA as the basis of an orbiting laboratory to be carried to orbit by the space shuttle.
The differential pressure capability of the basic 737 fuselage would have been fine for pure O2, partial pressure environment.