Strengthening the muscles, to my knowledge, will not of it self improve or decrease flexibility..
However if you don't stretch a muscle after doing strength work, it's almost def. gonna get tighter.
That said, some strength work will help improve your flexibility, like deep back or front squats...
The fact that your muscles are tight shouldn't be the determining factor as to whether you are going to work to strengthen them or not. The determining factor should be whether they need strengthening or not. And you say that they are weak. So strengthen them!
When you say you've tried stretching them, what does this mean? Does this mean, 1 stretch for each of them every second day for 10 seconds, after workouts? Or does it mean 10x day, 10 stretches, for 30 secs, for 10 sets? You gotta be more specific. What type of stretches? Relaxed, isometric, dynamic, static? A mixture?
I have yet to meet someone or hear of anyone that has had NIL success with a properly constructed stretching program.
Other things to consider-
Tight pecs or the appearance of tight pecs may actually be a weakness in the opposing muscles- rear delts, traps, scap, other upper back muscles etc etc.
If a proper stretching program fails to get you to a good level of flexibility, look into self myofascial release- foam rolling! Improving soft tissue quality can do a hell of alot for flexibility!
Hope this helps.