As you say Sidney, from what I have read nearly all, if not all of the Tracks in the USA are/were small. It is no surprise to me that Bast struggled over here. Over in the USA, Autrey, Hancock, Hamill etc. would probably struggle on those smaller Tracks as they would no longer be used to them, so the fact that he was beating everybody, including those mentioned, in the States is no surprise either.
Bast was a good, Rider in has own environment/comfort zone, but - to me anyway - he never proved his greatness in the different Leagues around the World.
Mauger, Briggs, Collins, Michanek were all truly great Riders, they proved themselves everywhere - I am afraid that in that pantheon of Stars he just doesn't measure up.
Mike Bast, good Rider, but in my opinion definitely not a great Rider.