Thanks to Richard for this comment on the Kirby Conan post:
Kirby also drew Conan on the cover of Marvel’s 1977 Calendar (along with a Kirby Howard the Duck!)
Here’s a scan of that image. If Jack did pencil this, Romita really stamped his own style onto this piece. Apparently Jack did some more work for the interior of this calendar. If any of you have a scan, please send it in and I’ll post it here. Thanks again to Richard for the heads up, I’d never seen this before and probably would have assumed this was by Romita.