they alow the motor to take in more air via intake and evacuate more air via exhaust. thus creating more power. in short the more air and fuel you can move through a motor EFFICIENTLY the more power...
A 355 is just a 350 bored 0.030 over. There really is no difference.
this is true but to further the answer if only you bore the cylenders out in the block you will need to buy aftermarket pistons...