Rem. might just produce the 700PSS in 300 Ultra Mag. you never know, but the PSS is currently MFG in what is known as the "short action" and the cartridge you are referring to would require the use of the "long" or magnum length version. The 223,308, utilize the short form. Not sure why you would want the PSS in the caliber when you can obtain it in 300 Win Mag. which is an outstanding long range caliber and that cartridge utilizes the Standard length action or 30-o6 length. I think I am right on the above, but perhaps others can add to this.
Good Luck