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The simplest set I found were from my local scrap yard. Look for a pre-1995 Mustang GT or LX with a v8 engine. The valve covers are made of pretty thick cast aluminum, and come with a tall oil fill tube and PVC port.
There are lots of simple chromed covers out there to choose from, I've got a set now on a motor in the shop that are Moroso's, but without the logo embossed in them. There's one vendor that's repoping the old 68 "Power by Ford" chrome covers too, but they run about $100 a set. My 331's got the 83-85 finned aluminum "Powered by Ford" covers that just happen to match the finned aluminum air cleaner top.