I don't recall what the fitting is on the back of the box. It's got wrench flats on it, it just unthreads. Don't lose valve behind it when you remove it.
Basically what you need to do is take your old pump, take the fitting off of it (this is the thread on/pressure side I'm talking about) swap that to the new pump, and you don't have to worry about the old style power steering box fitting...you continue to use the old flare style pressure lines.
I know on almost every vehicle GM changed the way the pump *low pressure* fitting exited the box, so I'm not sure if you will have a problem or not there. Usually not as big a deal as the pressure side since low pressure is usually mainly rubber, and thus more adaptable than the pressure side.