Gaining Upper Body Strength
Developing upper body strength is not just about packing on mass. It is also about developing isometric strength and muscular endurance. Granted, mass can be very helpful for boosting strength in the upper body but it is not the only way to gain strength. In fact, most mass building workouts are derived from bodybuilding which is more concerned with appearance than strength.
Bodyweight exercises are also well advised as they will boost strength and muscular endurance. Pushups performed with a variety of different hand placements (traditional, wider, narrower, etc) will hit the entirety of the upper body. Performing pushups slowly will aid in building up muscular endurance and isometric strength as well.
A word to the wise: always lift heavy weights with a training partner. This will make such sessions much safer.