Welcome to the Forum Bong.
Need to ask two questions first:
What caliber are you going to rebarrel in?
What do you consider Long Range?
Most long range shooters pick heavier projectiles. Heavier and Longer Projectiles usually need a faster twist in order for them to stabilize. That is why when they are flying and starting to stabilize, they may not be as accurate as a lighter bullet with a very slow twist in route at shorter yardages. The difference being that the latter's accuracy will fall off quickly after a given yardage while the former will improve as it moves farther away. This is why you see very different twist rates and projectile configurations between 100/200 bench rest shooting and 800/1000 yard.
My Palma Rifle for 1,000 yards is a .308 Palma with a 1:12 twist, but it is because some matches require 155 grain projectiles, while my TRF Class .308 is a 1:10 and the chamber is optimized for 176 grainers. My 6.5x47 Lapua and 6mmBR Norma are both 1:8, but my 6PPC and 30BR are both 1:13.5,
Then comes the next three questions:
What kind of accuracy are you looking to get to at what yardage? For some hitting a 9" pie plate at 1,000 yards is sufficient, not so for some others.
Are you going to reload? (Assume yes as that is a big part of getting long range accuracy)
Will this be for hunting or punching paper at long range? (Chamber Specifications will vary depending on this as you pick projectiles with different designs)
When you pick the type of projectile you are going to shoot, than it is time to decide Chamber dimensions including neck diameter (no turn or turn neck) and free bore.
If you want to build a true Long Range Accurate Rifle, there is a lot to consider other than just twist and we did not touch barrel length or contour.