A Stanford Torus and a Mars spacecraft are such wildly different systems they're not really comparable. You don't live in a Mars spacecraft all your life, for one, and a habitat needs shielding against the most hostile conditions the sun can throw at it during solar storms and such while a spacecraft can make do with a small storm cellar as refuge for the crew in the somewhat unlikely event a storm occurs during the mission.
Here are some specifications of a real world nuclear thermal Mars mission study: Mars 1994. It places the habitat between the propellant tanks which provide radiation shielding for essentially 'free'. I'm guessing the engines would also have a shadow shield which is a simple circular plate of metal in front of the reactors that provides shielding in, say, a ten degree cone or whatever is sufficient to cover the habitat.
As a curiosity here's an older yet more potent design that would likely be a political impossibility today... A fairly modest Project Orion vehicle that would take more more payload faster to Mars than even nuclear thermal rockets. This one is small as far as Orions go, and would have been launched on only two Saturn V's. (200 tonne total mass, contrast with the 800 tonnes of Mars 1994.) This would bypass the radiation and EMP issues that atmospheric use of Orion would have.