Gary Hudson later pointed out that such an engine existed, the Space Shuttle Main Engine, using a SSME-powered S-IVB as a thought experiment to demonstrate the real-world feasibility of SSTO launchers.
The Saturn C-5N was a conceptual version of the Saturn V launch vehicle which would have had a nuclear third stage instead of the S-IVB used on the Saturn V. This would have increased the payload to Low Earth orbit of the rocket from 118,000 kg to 155,000 kg.
The exterior of the Saturn V rocket was painted with titanium dioxide; this later allowed astronomers to determine that J002E3 was the S-IVB stage from Apollo 12 and not an asteroid.