One of the great things about a single shot like the Ruger #1 is cartridge length is no problem. Infact, a longer cart. helps me speed my reload when neccessary.
I have a #1 in .338x74K & love the round. With a long bullet like the 225grNAB, LOA is right at 4.00". You can easily get 2800fps w/ this bullet & the round is just very cool to load for & shoot. I recommend it or a straight 9.3x74R. If you are going custom, might as well go custom.