The problem with Pluto is they were using the technology for the wrong application. Once you get past the "low altitude flight," and other weapons requirements, and look at it only as a space propulsion unit, you get an entirely different kettle of fish.

You note that they found fewer radioactive products in the exhaust than expected. However, the unshielded reactor would kill anyone near it.

Put one of these in a large spaceplane with conventional engines that can take it to about 50,000 feet, THEN start the reactor to get to ramjet speeds. Radiation never gets far enough through the atmosphere to hurt anyone (use shadow shielding for the crew and payload). At some point, start dumping water or liquid hydrogen into this thing, and away you go.

The DUMBO propulsion concept was something like this.

More on nuke propulsion (including DUMBO) here.