If you going to mag feed it, install a Wyatt's Extended Box magazine. With a 6.5RUM you are going to be chasing the lands before too long, the extra mag space will be a HUGE help 300-400 rounds down the road.
With a cartridge like that, I would be very seriously thinking about rolling with a single shot follower. Then you can run any bullet at pretty much any length you wish. The 6.5STW is also a lazer beam, LTLR reported 3700fps with 130's, that's enough to make a guy go >>
Of course the 6.5AM shows 140's at well over 3500fps...