What kind of alloy are you using?
Yes, adding some tin might do the trick. I find if I go from pure lead to about 5% tin, it will increase the bullet size from .430 to .433
Just talking about molds, I cant recommend Accurate Molds highly enough. I got two traditional .44-40 bullet designs in the one two cavity mold, and it works and cast excellently. They had it to me in less than ten days as well, and I am a long way away from Utah.
