A kilocurie of Cs137 at 100 meters would expose you to about 23 millirems per hour. You’d need to stand there for 4000 hours to get a dose that would make you sick (not a lethal dose, just one that would make you ill. A 100% lethal dose would take 28,000 hours). An Alpha emitter of lower Specific Activity could in the long term be more fatal because of inhalation effects, such as lung and liver cancer. Thorium, for example, has a feeble Gamma associated with the decay and would be a little more difficult to detect and easier to shield, compared to Cs137 and Co60.
Th though has a specific activity so low that you’d need multiple tons to get the same amount of radioactivity in the air for people to breathe. That's why the Canadians modeled Am241, it has a "reasonable" specific activity. Anyone camping in Joshua Tree breathes Th, I ran a 2” pancake over the rocks and got 10 times the background I get in El Segundo beach sand. A spectrum taken later showed mostly Th.
You are correct about shielding, most of the actinides are harder to detect than a load of bananas due to their self-shielding. Nobody shields bananas, though, which have K40 in them, so a truckload of bananas or Brazil nuts looks like a pound or two of U or Th to the older NaI spectrometers.