Having read the script I think that, while there are a number of options, Michael K Williams is definitely the best fit for the role.
I want to level a criticism against the script, though. I think it will be interesting and enjoyable, but QT's mentality seems to be a total waste of his talent. He's definitely gotten into this "I'm such a fucking genius I'm going to make these weird little compelling 70sish movies" mode; he's really given up going for any sort of serious masterpiece since 1997. It's fun to have the 2000s QT of Kill Bill, Basterds, etc. but this will now be his fifth movie that is sort of compelling but not any sort of cinematic leap. Can anyone really say that we wouldn't be better off if these five weren't trying to hit the cinematic levels of his first three films?