I have a set of old M/T valve covers. Like the ones in the picture. I would like to get rid of the fins and M/T, and smooth them out to match the raised "inscription pad" at the bottom, for a custom look. I had thought about having them machined down but that would go into the recess underneath the raised pad.

Is there something I can fill in the area between the fins with? Too deep to want to use normal body filler obviously. I found "Metal Fill" supposedly cures hard enough to drill and tap, Bondo also has a metal filler. I am just wondering if whatever polymer base is in them will stand up to the heat that valve covers will being seeing on a street driven car.

Thanks all.