This is one of those times that you really laugh at how comical some gun laws are.
I looked at doing the same thing, but contacted Henry in the US to buy the stock, and they wouldn't sell it to me because of American laws on short-barreled rifles. There was a massive amount of red tape to get around, fees, a waiting list, etc.
So you can buy a handgun no problem in the US, you can buy the Mare's Leg no problem, and you can buy a rifle no problem in the US, but they'll be damned if you're going to buy a short rifle in the US.
So yes, we have some very dumb laws in Canada, but imagine being in the US and being able to buy a handgun without any problem, but not being able to buy a stock because it will make your Mare's Leg longer.