> you can shield a small portion of the spaceship with enough lead to create a safe environment.
Yes. Emphasis on "small." But there are better things to shield said compartment with than lead... water, food, structural elements, computers, etc. Lead is otherwise just dead weight.
> According to Robert Zubrin
While he is correct in this case... take everything that man says with a grain of salt the size of your head.
If they build a heavy ship and use it over and over the excess weight of shielding wouldn't be such a concern. They will probably want several so the mail service to Mars would have some regularity. Space Shuttles, the real kind.
See my post 82 for a link to a transport calculator that will let you see how much lead you need.