Best way to clean valve covers?

buy a can of paint stripper or graffiti remover and drench them in it. let it sit and break all that stuff up for an hour (the can will say 20-30min). then use abrasive tools such as wire brush, scotchbrite, etc. the stripper breaks the bond and everything will slide right off. you might need a dremmel to get into the bolt holes and the lip.
you might not need high temp paint for them. i painted my buddys with a couple coats of regular grey primer and then with gloss black krylon spray paint. i put on about 5 coats and wetsanded between each coat. his mustang is a '68 and is not driven everyday. the paint has held up but it might not work for a daily driver in hot weather.