mine is a big heavy match rifle not a hunting rig. I had a huge internal struggle over barrel length, finally chose a 26" but in a hunting rig I'd say 24" max. The reloading manuals mostly all call for 24" so there you go.
Jerry Newman built my rifle and a host of other 260's as well; PM me if you want his number.
Not sure about bullet selection. My top choices would be the accubond (130 or 140), the 120 ballistic tip, a 130 scirroco II, or a berger 130 or 140 if you can make them shoot. If you're in a lead free zone, the 120 TTSX or 120 GMX (or 95gr V-max only for condors