Remember, it's a Federal crime to remove or tamper with emissions control equipment. The individual States also have rules forbidding this type of tampering.
Bottom line, although you don't need to smog test pre-1974 vehicles in CA, you ARE required to have all of the original smog equipment installed and operational, and if you do an engine change, you ARE required to have it Certified by the State. The rub is this: Without having to take it in for a smog test, there really is no enforcment protocol in place to catch you. So, unless you get stopped at a roadside mobile emissions checkpoint (very very doubtful in CA due to the current Budget crisis), or get pulled over and the Cop gets wise and looks under the hood (also somewhat unlikely), then you are in the clear. Just be aware that the Federal penalties are very severe ($10,000.00 plus jail time)
Tim