Yes it "will" fit a standard length, this is why I built it, to fit a standard length action.

Quality Cartridge can make up brass for you. They get their basics from Mast. But when I talked to them last year about nonrebated RUM brass, they conceded reluctantly that their RUM brass was Jeffery resized, i.e., undersized at the base by about .006" ().
Horneber also will do a custom run of whatever you can dream, more or less, but that's serious bucks. IMHO the good part about your idea is that you can use RUM brass off the shelf. If you're looking into custom brass, you're much better off going back to the excellent Jeffery case and Dakota cartridge.
If you want to do this 300Win-length RUM, find a smith and get them to talk to Pacific Tool and Die, or any of the other reamer makers, CH4D can make sizer and reloading dies for you, and off you go. While you're at it, feel free to improve the RUM case to .532" at the shoulder, 35 deg. You should get about 103g of water into the case, as opposed to the 86g of the Winchester or 110g with the RUM. The Dakota has around 97g, but keep in mind these are all case capacities, not necessarily directly comparable. Different neck lengths and case-length to COL relationships will effect the real useable capacity of each.
Again, think 300 Weatherby performance in a standard length, and you'll be in the right ballpark.