I hear you don't want to get none 7-08 head stamp brass. However that is all we are talking about 260 rem is very easy to open up with a straight mandrel 1/2 a mm only ! To size down 308 is just 1mm . By rights we should size all new brass before ever loading it anyhow , so we are talking just the head stamp !
As an ex 7-08 owner , the issue is and always will be availability and the extra cost . Don't handicap yourself your brass wont perform better or worse from it being run the first time once ( indeed its std practice to re-size all new virgin brass) the tiny move of this makes no matter at all )