A better choice in determining cast bullet diameter is to use the biggest diameter that will chamber, that would be one that will just fit in an unsized fired case, cast bullets can't be too big as long as it can be chambered, they'll size down to the groove diameter when they're fired.
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I'd go with Montana before Beartooth if you want to get the bullets timely, I've ordered lots of bullet from BT, excellent bullets, about half the time they have taken a month or two to arrive. I've not ordered from Montana myself, but I've only seen glowing reports for their service and products. Mt Baldy is another good one I've dealt with as is Western which is fast most of the time, but not always. There are prolly 40 or 50 cast bullet sources listed in a sticky in the Cast Bullet forum here on GBO.
Tim