By Federal law, the upper is not the controlled part. However, if you own both a rifle lower and a short-barreled upper, you would be in "constructive possession" of a short-barreled rifle.
The actual legalities of a Canadian citizen buying it, I have no idea.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.