Do you think before you purchase things? If I am not mistaken the B44 uses SAE style nuts and screws just like the Jconcepts BJ4, since there happens to be alot of aluminum on the 501 that is already pre-tapped for the metric ball studs, b44 ball studs (which are 4-40 sae thread) will not thread in. (unless you want to take a 4-40 tap and risk ruining the threads just to try to fit the b44 studs)
I suppose you could use a b44 ball stud on an area where it will need a nut holding the stud on, but then you will need to use a 4-40 nut. And if you are going to have to use a 4-40 stud, why stick to the b44 ones? Might as well upgrade to Losi or Lunsford titanium, but I wouldn't bother--I would stay metric with one of the Japanese manufacturers. If you are determined to not use Tamiya ball cups, I have heard the Kyosho cups fit pretty well and also some guys are using the Yokomo Mr4bx ball studs--although I don't know if you must use Yokomo ball cups with them.