Not likely

In his excellent, infuriating, and engrossing book The Song of the Dodo, on page 65, David Quammen describes Alfred Wallace's observation that monkeys, unlike tapirs and other animals, don't swim (a key point in the recognition of biogeographic principles). Quammen adds that monkeys don't walk much, either, spending almost all their live in the trees, and says

They wouldn't likely cross the Amazon River even if it were a great brown highway of dirt.

Two things. Well, three. That use of great is odd to my ears, very British. (Quammen is not; he was born in Ohio and currently lives in Montana. He did go to Oxford as a Rhodes Scholar; maybe he picked some things up there?) Also, I note he uses the modern standard subjunctive, a nice compromise between the older 'were it a...' or the modern colloquial 'if it was...', though in this popular book the latter wouldn't have been out of place. But mainly... wouldn't likely cross?

Does that use of likely seem okay to you? I certainly don't have any trouble understanding the sentence, but my usage of likely is mostly confined to be+to-infinitive constructions. I'd say:

They wouldn't be likely to cross the Amazon River even if it were a great brown highway of dirt.

Of course, what I'd almost certainly say is

They probably wouldn't cross the Amazon River even if it were a great brown highway of dirt.

And if I had to use likely, it would go just where probably does.