I have a 96 k1500 with a 350 vortec. At start up engine light comes on with crankshaft positioning s sensor code. If I turn this light off while truck is running it doesn't return back on even after running for long periods of time but returns on at next start up. I have replaced the sensor and the light still comes on. This truck has hard start, sometimes no start also runs extremely rough. At start up if I do not rev it up a little it will die.

my only suggestion was to change sensor but u said you already did that. I had the same problem with the rough starting just the other day. I have a 97 GMC with the same engine and it just started hard and i couldn't keep it running without having to keep the engine reved

It still could be the sensor. Not all sensor manufacturers get it right.
See if the parts store will exchange the defective sensor for one made by a different manufacturer.
Actually, what I would do is return the defective sensor, get my money back, and get a new sensor from GM.