Since we have information from
MJ on what he was thinking when he designed the Klingon battlecruiser by 1967 it would make more sense to just lean into that "future" design thinking.
There are 26 episodes before "Errand of Mercy" and looking at all the alien and non-Enterprise ships (excluding shuttle Galileo) created during that period there was only
one ship design that had nacelles: the impulse-driven Romulan warship. Mudd's Class-J, Lazarus' ship, the Fesarius and the Botany Bay all lacked nacelles.
If the series was pressed to come up with a Klingon or Gorn ship in Season One I kinda doubt they would have had nacelles based on what came out during that time. YMMV.