No way to prove it is an original 409 car unless the owner has the original cracked block. 409/340 HP at best since the car is an automatic. Car was unfortunately most likely a small block car from the factory as the fan shroud is not 409.
GM Canada docs available will show the original engine, etc for any Chev or Cdn Pontiac from 1955 on, they also stamped blocks with a number from 55 on. 1964 on you can get a build sheet with all the options. My 63 was built as a 283 powerglide, I have the paperwork on it.