Chilled #6 would be pretty close to WW. Magnum or premium is a good bit harder. That trace of arsenic is no more dangerous than the lead in the quantities involved, but it is needed if you were waterquenching or heat treating.
Half and half shot and lead oughta work fine for what you are doing, but it might need some tin for better fillout. I find it easier to just add tin as a matter of routine than to make ingots and find out later that I needed to add tin. One pound of lead free solder or two pounds of 50/50 should be enough for 100 pounds of alloy. I use about twice that, but I work at a lot higher velocities than you are.