Is your 305 a early, LG4, later LG4, or an L69?
1986 LG4
Considering they were produced from 1980 to the end 1987 (and one production run was done to have them for the 1988 Monte Carlo and Caprice - but there were no LG4's Actually Made in 1988) I would say a Late LG4.
-Gonz