Easier to do with new brass. 264 would be the smallest resize, but it's not the easiest to come by either. Probably the most common will be 7 Rem Mag, make sure you get some Imperial Resizing Die Wax (or whatever they are calling it now days), makes the job go much smoother. If you don't have forming dies, neck the 7mm down to 6.5/264 first, then from that down to 25. You'll have to set the shoulder back to fit the Wby chamber. Trim to length, if required, then measure the neck thickness and diameter, inside and out. If that's ok, you're ready to load, if it isn't, you'll have to turn or ream as required. Load with a starting 257 load and fireform, then check how it all turns out. - dan