i know the feeling on the money deal. luckily i only have about a hundred in mine so far on the build. the 89 will have the 4 separate headlights. i believe 88 and 89 were the only two years then in 90 they went to the plastic one piece lights. check out this link i found. it gives some good history on the gmc/chevy trucks.
http://www.edmunds.com/gmc/sierra-1500/history.html