i like mustang's ride for most of the time, quite soft, but on some bumps(in which there are a lot of them in boston) the car is really bouncy. Especially in the rear I guess. I've been thinking of lowering my car, would a lowering spring help in that sense? or should I change something else?

Thanks

 

Lowering would make it worse, most likely. The problem is 1) solid rear axle tends to make the bumps more noticable, 2) rear wheel drive means more unsprung weight on the rear axle than other front wheel drive cars. This makes every bump more noticable.
You could try different tires, with a softer sidewall maybe...but that would detract from the performance of the car.