Not sure, Andre...
As you know I wasn't living in the States back then so I don't know for sure but I've heard America discovered the smurfs thru the cartoons which I think came out in the early 80s
This would also correspond with the release of the smurfs made specifically for the American market (Cowboy, Indians, Baseball, Football).
When I was in Germany last year I noticed how many Bully figures they sell in the toystores..but you never see them in American toy stores (at least I don't), so I don't think Bully smurfs were wildly sold in the US stores, however, I would not be surprised if you could find them in specialty stores like comic stores.
Hopefully, somebody who remembers the 70s will be able to tell us