If you must save money on the mold, a 375 mold could be sized down and may give acceptible results. If you go that route, anneal the gas checks and install on the bullets before sizing.
However, measuring the inside of the fired cartridge neck is not a good measure to fit cast bullets to, as chambering will probably be a problem, with bullets hanging up in the throat. Understand that when the powder applies it's peak thrust the bullet will be upset tightly into the ctg neck, even if it is sized to a much smaller groove or throat diameter diameter, and this upset can hurt accuracy considerably, especially if the bullet bases are out of square during sizing. So again, if you opt to size 375 bullets down till they fit, be sure to bump the bullet bases square at the bottom of the sizer stroke.
For sure results the first time, make a throat slug per my catalog instructions and send me an order. I'll make a custom mold that WILL fit your rifle, and WILL shoot accurately on the first go around. With all molds like this I hand write specific reccomendations regarding sizing etc which takes most of the wasted experimenting expense and money out of it for you. My molds are expensive, but in the long run you'll probably save, and most important you'll be a happy caster, not a permanently soured one!
