296 has LED rear lights, 276 has regular bulbs
296 doesn't have an orange reflector strip on the side of the headlights, 276 does have reflector strip
296 has fewer and fatter horizontal bars on the front grill than 276
Not sure about timing covers
313 has all the features of the 296 but has a totally different engine and a bulge in the bonnet to accomodate the different (and taller) engine.
276 and 296 have single air intake, 313 has twin air intakes
276 Front
http://image.automobilemag.com/f/auto_shows/6688712+w440/2003_ny+2004_nissan_350z_roadster+front_left_view.jpg
296 Front
http://image.europeancarweb.com/f/8758922+w750+st0/caep_0712_03_z+2006_nissan_350z+front_view.jpg
313 with bonnet bulge
http://stockimages.autolocate.co.uk/imageor404/101351_654_2872.jpg
Hope that helps