If your wheel weights are thirty-five percent tin and antimony, and you want to reduce that percentage to, say, 5%, you would have to mix the weights with 7 times the weight in pure lead. 4% would require a mix of 9:1; 3% would require a mix of 12:1. etc. By the time you are using that much pure lead, what would be the point of using the Wheelweights in the mix at all? Use them for pistol and modern rifle bullets, or trade that stuff to someone for pure lead. When I was a teenager, my father wanted to cast some hard, .357 173 grain, Keith Semi-wadcutters to use in his revolver. I did the casting for him. It was difficult to get the mould hot enough and the lead hot enough to cast good bullets. But, when I got them, he found out they were so hard he wore out his arm and shoulder working the arm on his lubrisizer. The bullets would spatter on steel plates, but if they hit anything else, there was very little deformation. When we used the same mix to cast bullets for his .45-70 rifle, the accuracy was lousy, because the bullet was too hard to expand and fill the rifling. Years later, we found that a 1 part tin, and 20 parts lead alloy was the right mix for those .45-70 rifle bullets. NO ANTIMONY!
Sorry if this was not the answer you were hoping to hear. If you are just going to do a lot of plinking with your ML rifle, then go ahead and cast the balls out of wheel weights. You will find that the diameter of these hard balls will be smaller, and you might have to use a thicker patch, but for informal shooting, they will be adequately accurate.
Good luck.