I found some interesting information on this web page (http://www.hotrodscustomstuff.com/legislation.html) about vehicles with engine replacements. The relevant information is about 15-20% down the page. The third paragraph under the Californa section is the important one for most of us, and it specificly refers to crate engines.

thats why i would only buy vehicles 1976 and under smog laws suck

smog control = hippes id rather punch a whole in the ozone everytime i start my car