I think you got the direction reversed. Here you have a reduction of B(x,y) to the halting problem, which only shows that (a subset of) halting is at least as hard as B(x,y). You need a reduction from collatz to halting to B(x,y) instead. Collatz to halting is trivial, halting to B(x,y) seems a bit less so - you need to precisely define which subset of halting problems can be reduced from Collatz yet is no harder than B(3,3).