switching from a vertical to horizontal scroller

The only idea I can offer is you have to determine what boundary controls are used and set them relative to the _width of the scrolled item.  It sometimes takes a bit of head and pencil scratching to be able to visualize how to generically define the parameters.

For instance, if the left edge (x=0) of the item is at x=0 pf the parent, then you know the limit for scrolling right is that x has to remain <= 0.  But for the other end of the item you want that to be stopped when its right edge hits the left edge of the viewable area.  That means the 0-edge of the item is almost the full _width of the item negative except for the width that is left showing.  So if you're scrolling to the right, the scrolling ends when x > 0, and if you're scrolling left the show stops at x < -1xitem.width + viewingArea._width

Scratch your head over that... it's hard to explain, which is why a pencil comes in handy at times.