8 twist 243 is what you want/need, you will regret going with a slower twist.
We killed a LOT of big deer in Kansas and Nebraska with a 243 using 100g Hornady BTSP, shots were usually between 200-350 yards, Rem 700's with 4x16 scopes, rattled them in. 6 Rem in is no slouch round either.
The 95g Nosler partition is the bullet for 450 yard and under shots in my opinion, as they will break shoulders on 260 lb bucks on quartering shots and exit on the off side. I have no experience with the 95g Nosler ballistic tip.
Having said that, 600 yards in open plains country, a 7 mag with a 162g sst at 3130 is our choice, due to wind bucking ability. Rem 700's, tweeked, bedded, freefloated, good trigger, solid stock, muzzle break will shoot 3 1/2" groups at 600 yards, the LSS version in particular is our favorite.
I really don' t like the idea of a 105g bullet hitting a shoulder at 600 yards, and you can never know what angle you will have to shoot an animal from. Also, bullet do some strange things when they do hit shoulder blades and big bones, and very often there is deflection involved. A 260 Rem with a 140g SST at 2950 would be a winner as these bullets are tougher than you think, and will penetrate on quartering angles breaking bones on the way.
Having said the above, I have an old friend that moved to New Zealand. He swears that the 243 with the 95g SST is all the bullet you will ever need, and they shoot those large 450 lb deer down there from mountain side to mountain side.
Never think of live game shooting as always having the perfect broadside shot, never.
We hunted elk off of horse back in Az. We could ride up close to the elk and all they ever saw was the horses. I used a 243 on cow elk with a 95g partition, never lost one and never had a goat roping tracking job either.
R#26 takes the 243 to another level, 100g bullets at 3200-3300 fps.
600 yards....... can of worms....no kidding......howling wind, cold dense air, up hill or down hill shooting...always use enough gun and know your limitations. I hope you will have enough scope where you can tell if the buck has broken brow tines off fighting so you can pass on him.