If you are going to worry about the twist with a new rifle, you will talk yourself into blowing $500.00 on a new barrel without giving this one a chance. The time to worry about proper twist is when your barrel will not shoot and must be replaced. Your barrel should handle a broad range of bullet weights.
If you are going to use the magazine you will have to keep your OAL consistent with a length which will move freely in the box. Depending on the length of the throat, this may not give you much latitude to adjust the OAL. If you are not concerned with using the magazine, work up a load which has the bullet firmly seated in the lands. I like this approach for two reasons. Firstly, a rifle's best accuracy is often displayed when the bullet is seated right into the lands, and secondly because I think it's safer. If you reduce bullet jump with a load that is already hot - but safe and accurate with bullet jump - you could end up with pressure problems as you increase the OAL.
Now you have a load which gives you good accuracy with the bullet seated firmly in the lands. Start reducing the OAL until you find the sweet spot which gives you the best accuracy. If you are patient you could reduce the OAL by as little as .0005, if you are not adjust by .001. Once you find the sweet spot, it is simply a matter of tweaking the load slightly to find the best accuracy for that OAL.
All the powders you listed will give you good accuracy with the .308, but pick one and start with it to keep the variables manageable. I like Varget, but everyone has their own ideas. I would also pick one bullet and start with that for the same reason. If you plan to shoot at medium ranges the 168 gr. will do very well out to about 800 yards. If you are bitten by the long range bug, then 155's, 175's, or 200's are a better choice as they stay supersonic longer.
