[Mr70]

I don't think there will ever be a Definitive answer on this subject.
Last year on Team Chevelle there was a great topic about this very subject.Almost four pages of people weighing in with what that have encountered.
Some Chevelles even supposedly built with one plastic,and one Steel.
I think the Leeds Plant mostly used Steel,and Van Nuys and Arlington were more favored with Plastic.
Funny,when owners said they were restoring years ago,they would replace their Original Plastic with Steel,cause the Plastic cracked after so many years,and Steel owners would replace with Plastic,cause they rusted or dented after years of use.