I used SPCN this year, so I can't say from personal experience that you can definitely use an old block to pass smog. My statements are based on my, admittedly, layman's understanding of the laws.
From the CA DMV web site:
http://www.dmv.ca.gov/vr/spcns.htm
"Specially constructed vehicles do not have a manufacturer assigned model year because they are esentially home-made vehicles. In the past these vehicles were subject to the emissions control requirements applicable to the year of the engine installed in the vehicle and the regular smog inspections per the county where the vehicle was registered. The year of the engine was determined by a Bureau of Automotive Repair (BAR) referee during the initial inspection.
In 2001, Senate Bill 100 passed which allowed the first 500 specially constructed passenger vehicles and pick-up trucks presented for original (initial) registration to DMV to have the year model for emissions control equipment and inspection purposes based on the appearance of the engine
or the vehicle as a whole. In addition, if the vehicle or engine does not resemble an established make or year model BAR is required to assign 1960 as the year model for emission control purposes. In 2002, another Senate Bill passed, SB 1578, which allowed previously registered vehicles to be allowed this special consideration by BAR in addition to initial registrations."
To me that indicates that participation in the SB100 SPCN program is voluntary and not required. If you are not one of the first 500 vehicles and do not receive a SPCN sequence number, the old methods of registration are still available.