No valve covers, just a rant and a possible solution. I called RPM a month ago, and they said the same thing. They also said they were re-doing their website and the unavailable parts would not be listed on the new site. Well the new site is live and every part I asked about, which they no longer make or sell, are still listed.
Now while searching I found that the GM 231 V6 uses the same valve cover gasket as the Brodix heads. Those covers can still be found new and used. They might no be tall enough, or they might. If not, it may be easier to modify a set than to find a set of sheetmetal 'race' covers. Good luck