From the title, I thought this was in reference to a car powered by a radioisotope thermoelectric generator, like the kind used in spacecraft and some lighthouses. But the article mentions being powered by uranium pellets, and wikipedia confirms "The car was to use a steam engine powered by uranium fission similar to those found in nuclear submarines"
They actually envisioned having a true reactor under the hood.
RTG powered car could work, but the electrical power available would be rather small, they have horrendous power to weight ratio. Steam turbine powered by heat from the radio isotopes would work better, but would still be very very expensive due to cost of the isotopes.
An actual fission reactor generating steam would indeed be the best most likely, when you ignore small stuff like emergency cooling, all the control mechanisms and shielding. ;-)
They actually envisioned having a true reactor under the hood.