Best of both worlds in your case, don't know what the price for a Nagant is over there but I,m guessing cheap, I wouldn't buy one just to do this though .....
I suspect the single stack magazine of the Ross MkIII would be the most likely to work reliably with 7.62x54R, but I've never actually tried!
“There are invisible rulers who control the destinies of millions. It is not generally realized to what extent the words and actions of our most influential public men are dictated by shrewd persons operating behind the scenes.”
Found this one the web. Apparently some from Santa Fe New Mexico (USA) has done conversions.
"For those interested in having their .303 SMLE convered to 7.62x54R, Contact Bill Black at: Santa Fe Gunsmithing, Inc., 509 Airport Road, Santa Fe, NM 87505 505.438.4174 The base charge of $160.00 includes the following: Pre/Post conversion test firing; Rechambering; Magazine modification; Muzzle recrowning; Return shipping via UPS standard ground. For an additonal $60.00, Bill will refinish your rifle in a dull bead-blasted gray/black parkerizing, a matte bluing, or moly-resin coating." http://jpfo.org/filegen-n-z/smle.htm