There is a guy reproducing those in fiberglass, he sells them for $75 on ebay. They look just like the originals.
They fit on the super turbine 300 and fit buick/olds/pontiac. I would call him and confirm, but the one he makes should fit your 326/SuperTurbine300.
Search on Ebay for: Buick Nailhead 401-425 st400 and st300 flywheel cover and it will come up.
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1969 Camaro/ Tom Nelson TT 434 / Wayne Due C5 / DSE QLink / and a bunch of other stuff...