As to the engine code of GJ, I believe your assessment above is correct if you are looking for beyond a shadow of a doubt confirmation. As there is not a published engine code of GJ for a 1962 model year full sized Ford made for the Canadian market.So I am kind of up a tree for finding out what it is at this point of time.
Recap for full sized Ford Mercury built for the Canadian market in Canada
Model year 1961 has a different format for engine codes (letter number)
Model year 1962 has the correct format (letter letter) however GJ is not published
Model year 1963 has a published engine code of GJ, however not for full sized Ford. Only full sized Mercury.
Model year 1964 has the correct format (letter letter) however GJ is not published. Similar to the 1962 model year,
What was built originally is cloudy
It is a FE V-8
Could be a 352 or a 390
It is definately not a 406