WW is likely too hard, if of the cramp-on type. The stick-ons are usually pure lead, or zinc - do not use zinc.

Zinc are easily identified by colour and feel & many have a Z on them.

50/50 WW (Canadian) and pure lead is a good mix and will slug fine - about brinel 7 or so, maybe 8.

I would anneal that brass before firing it.

Small rifle primers have longer cups than small pistol primers. CCI450 magnums are what I would use, or Rem. 7 1/2's - both of these also have the thickest cups.

Good luck, have fun. OH - if you can find some steel shot wads, cutting the wads off just above the WEB, will give you a cup - that will hold an undersized ball perfectly centred in the bore.
With black powder, you should have a seal (over powder wad) between plastic and black powder, as the BP will melt the plastic and coat/foul the bore.
The use of a barrier wad will eliminate this. The plastic wad will 'take' the rifling and give the ball a spin, just fine.
I would first, soften the alloy, then try them for it.

A wad with a hole, like an overpowder card wad or cushion wad with central 3/8" hole (doesn't have to be deep), set on top of your powder wads, will also hold the ball in the middle of the bore, for improved accuracy.

Good luck, keep us posted.