My .300 Tomahawk has a 30" Hart barrel that tapers to .950" at the muzzle. I has the 5.5-22x56 NXS and I have a McMillan McHale stock on order. I think you will really like the setup you have planned. I also have a standard .300 RUM built by Jerry Rice which has a NXS and a McMillan A3. Do yourself a favor and try the 200 gr Nosler Accubond when you get your rifle built.
Your plans are right on the money. I would stick with a 26 or 28" barrel, especially using a Mike Rock or Obermeyer 5R barrel. They will give you the velocity of a 30-32" regular barrel. This way your gun will still be managable in weight and portability.
And it will get you to 1000 and then some.
I would get a Hart Max Heavy Varmint Taper barrel 28-30 inches long.I currently have 2 on Remington actions and no problems at all with the bedding.If you choose a 1.200-1.250 or larger straight barrel you might want to consider a barrel bedding block to put less strain on the bedding.The other good news with the Hart is that Lester Bruno usually has one in stock so no 3 month wait as is typical with the custom barrels. www.brunoshooters.com once there click on store then scroll down till you see barrels and click again.
95.5 gr Retumbo behind the AB for 3200fps from a 25.5" cut rifled K&P barrel. I think its right up there with the 5R barrels for velocity.
My Tomahawk(.300 RUM Imp.) has a 30" Hart and it does 3200 fps with a 240 SMK and 106 gr H870 -- Not too shabby on the fps, either...