I have a Winchester Modle 70 in 264 Remington magnum. When I was reloading for it I could barely exceed Winchester .270 veloitcy. Given the huge quanity of powder that the .264 takes to achieve velocities of a 30-06 based case, I would most likely choose the 6.5/06. With your long barrel and a proper secection of slow powder you will get plenty of velocity.
Clifford L. Hughes