Ah, ok - ignore my previous post about the spacers, since you have a screw on freewheel. If you had a freehub (where there is a cylinder that the cassette slides onto, like this:
), what i said applies (you basically use the spacers to fill up the space where the cassette used to be, and when you tighten the lockring the tension snugs everything up).
However, what Dobber says above is correct - you're probably not going to need anything more than the bmx freewheel and redishing. Which is better in the long run, since your wheel will be more symmetrical and thus stronger.