Wierd sizes.

I might think along the lines of a necked-down .45 2.4" case. Purchasing a "chucking' reamer for the chamber's neck can cut costs of a neck throater, down to just a throating reamer. Anohter idea is perhaps they or you could grind another groove diameter sized chucking reamer with a 3 or 4 degree angle for the throat. It would need a pilot. That work is beyond me, merely a suggestion of what 'could/might' work.

Just trying to cut costs, as usual.

With the above, your existing .375/2.4" reamer could be used with a proper sized pilot to cut the chamber.

A mould for the 12" twist would be anything from 300gr. that would run about 2,400fps to a 400gr. at just over 2,000fps or 2,100fps. Paper patched would easily allow full speed loads without using anything harder than WW alloys.

A lubed, paper patched bullet should give up to 100fps higher speed over a jacketed bullet's velocity, same load.

For high speed loads, the paper patch bullet mould should cast right at .388" to perhaps .390" max., I believe.