I too have been thinking about 30-40US. Its one of the cartridges Id like to see added into the lineup, along with 9.3x 74 (Id even accept 9.3x63) AND 303Brit.
Good luck with your conversion. For me the building is as much fun as the shooting. Im currently waiting for my 45-120 Buffalo Classic to arrive. Ive added a 30 inch brass scope and had the barrel lever "rolled over" into more of a side lever design and changed the trigger guard into a larger "schutzen" style.
Soooo, get excited about your 30-40 conversion. Do what you want. Seems to me Fred had a heavy barrel turned into a octagon in one of his specials.
In my mind 30-40 is a great cartridge thats still available in most gunshops and fits into the catagory of unique. Its not something your buddies will show up with in camp, but its clearly a killer down rang without being too hard on your shoulder.
GO FOR IT