I own a '91 Sierra and i'm a huge fan of those cars! U have some mistakes on Your car...i'll try to point them in pains in few secs
Anyway nice start and great choice of the car
EDIT: the green line shows the edge being to sharp and the red line shiws the place where it should stick aut a bit more...it would be the best if U check some more ref pics to see what i mean
the overall car shape hasnt change that much over years....the front ant the rear parts changed most
Good luck