I have some buddies that have experimented with different twist in their 1000yd BR rifles. When I fired mine in competition I always used a 10 twist. That is the probably your safe right down the middle of the road bet for all 338 bullets. But some have successfully used looser twists with varying degrees of success. But this is for BR competition when your lookng for every advantage. If this is going to mostly be a hunting rifle I would buy a 10 twist and not even give the twist another thought.
As for barrel length, that is very subjective to your application. That is really your preference. It will be more efficient with a longer tube, but if efficiency isn't what your looking for then you can throw that out the window. But to give you a quick reference guide to make your decision.... here is what I beleive:
26 to 28" - will work just fine. you will probably loose some velocity but if your rifle is for a hunting application with a sporter type rifle, 50-75 fps +- is nothing to be concerned about. That why your scope has clicks in it.
30-32" - is probably a good compromise for all around efficiency and portablity.
> 32" - is for specialized long range rifles that are made to balance out with the longer barrel and get maximun velocity for the extra long shots. More money, tuning nodes are different, etc, etc. Like I said very specialized and there is always a price to be paid when going down this road. By that I mean a price from a money and time/learning curve perspective. You don't always hit the ground running with a new rifle in this configuration.
All in what you are looking for.