The stamped steel valve cover continued through 1980.
The major engine changes were for 1981.
1979 and 1980 got the iron 2V manifold with the BBD that was introduced in 1977 for the trucks and wagons.
The revised engine got a plastic valve cover, lighter crank (fewer counterweights), aluminum intake manifold, lighter/smoother cast iron exhaust manifold, electric carb heater in place of the exhaust flap.
Presumably the block casting was updated too.
The plastic valve cover leaked worse than the steel cover.
There was a fancy/expensive (ca $50 in the day, NLA AFAIK) laminated valve cover gasket that Felpro released to address the problematic leaky plastic cover.
Many owners switched to a cast aluminum aftermarket cover to address the leak problem.