I put on some really, really heavy cast-aluminum valve covers with built in o-rings from a 502 when I re-did the intake manifold gaskets. (My mechanic lent them to me as he had bought them for a project and then never used them... but they were worth too much $$ for me to buy.) I had installed some cheap (still ~$100) stamped aluminum ones last year and wasn't convinced they weren't leaking.