You may want to establish this car as a 'historic vehicle,'
(manufactured between 1922 and 1981.)

< http://www.dmv.ca.gov/forms/reg/reg17a.pdf > --yellow on black plate
number gets entered on the form then you certify its historic status in
section E.

All driving will be on the way to shows, parades and club meetings.  You
just started several days early (:-) and it's a long distance to get
there! If a CHiPs starts camping out in front of your house, you know
the game's over.

No sweat on the duplicate number problem if the old plate you get is a
legitimate plate (was made by CDC).  The "new" plate design (post late
'60s) doesn't duplicate any numbers used in earlier series of plates (my
memory on this only goes back to 1938.)  Only exception would be if
someone used a yellow on black number to make a vanity plate -- not likely.

If you live in SoCal and are an auto club member, go there instead of
going directly to DMV.  The Auto Club license clerks are really great
and many times know how to get the job done whereas the average DMV
counter clerk only knows how to say, "no."

Run this past the Auto Club:  Register the car in Califonria now, in a
non-op status.  You'll get a current series plate.  Then, when restro is
completed, re-register as a historic vehicle with the yellow-on-black
plate.  And, as Wayne suggests, if that was a California plate
identified against the VIN --I'm not sure that we had VINs then -- maybe
it was the block number), you'd be in clover.

I occasionally see black-on-yellow plates around town -- that series
preceded the yellow-on-black.  Either these people are breaking a law or
the system will accommodate those old numbers.

Best of luck.