Hello, you will find the engine number of a 409 under the alternator, on
the front left side of the block (looking at it from the front of the car),
like you can see here:
http://www.stefanopasini.it/Bristol_...ne_Numbers.htm
Removing the alternator and cleaning/painting the metal of that small part
of the engine block will allow you to see it clearly.
As far as I know, the engine number has no relation with the chassis number.
Cheers
Stefano