Wouldn’t that be just the way…
I would use a flex (cutting wheel, hand held tool?) and later files and a belt sander and build a jig or at least a profile, measured out nicely, so the symmetry works out well, but I would be worried about strength, especially at the bolt holes, and also concerned about making the connection to the front part. Depending on where you live a few hundred bucks for rough, smooth ones might be worth it. Probably not, for your build. I‘d do it inside with something to catch as much dust as possible. On the other hand you could also keep the ribs for your project and embrace them.
Yep, I think I’ll keep the fins as-is and an eye open for a pair of smooth ones. I have many MANY more places to put that kind of effort than into valve covers!