Incidentally, it's probably a good idea if you limit the field-of-view of the IR sensors as well. Either use a lens, or better yet if the emitters are sufficiently bright, just have an aperture a good distance in front of the sensor. If you have a 1-2 mm hole a cm or two in front of the sensor, lined up with the emitter, you'll have a really well defined sensing 'beam'.
If you want an 'analog' solution, you might be able to find a linear PSD(position sensitive diode - it's a large photodiode with multiple taps, google it) and adapt a scanner optics assembly. The digital version is to just *use* the scanner, complete with CCD sensor and use a pair of them as the sensors.
James