the offset on the button flap stem is slightly different between the two shells..
Just file it down until it feels right.
You have to lightly attach the screw each time to test, otherwise the feel is different with and without the screws on.
for the wheel, just remove that spring on the right, it's not necessary, the switch is enough to push the wheel back up.
However, the reason that the spring is there is because the original logitech wheel slider mechanism is so heavy, that if you picked up the mouse and dropped it down from too high a distance, for example, quake 3 quick swiping.. Then the wheel might click because of the momentum of the slider, IF the spring is removed.
But with the spring removed, the wheel is much softer to use.