I would think a 1 in 8" twist ought to pretty well cover you for most bullets in that barrel length. Mr. Alexander chose 1 in 7.5" for the short barrels (10.5") to reflect their lower velocities. You would probably even be fine with a 1in 9" twist, as I know some of the other manufacturers use it, and I don't recall anyone having any troubles because of it.
If you plan on ever using a suppressor and firing heavy bullets at subsonic velocities then I'd likely opt for 1 in 7.5" like the shorties use.
I can't help on the other parts, but I'm sure the more knowledgeable chambering folks will drop in soon.
ETA: barrel length-- Are you keeping it as a hunting rifle, or going heavy? That will likely somewhat steer people's opinions on what is the better length. Big and beefy? I'd go 22". Svelte and going game stalking? I wouldn't go any longer than 20" (perhaps even shorter depending on it's intended usage and expected distances).
If you plan on ever using a suppressor and firing heavy bullets at subsonic velocities then I'd likely opt for 1 in 7.5" like the shorties use.
I can't help on the other parts, but I'm sure the more knowledgeable chambering folks will drop in soon.
ETA: barrel length-- Are you keeping it as a hunting rifle, or going heavy? That will likely somewhat steer people's opinions on what is the better length. Big and beefy? I'd go 22". Svelte and going game stalking? I wouldn't go any longer than 20" (perhaps even shorter depending on it's intended usage and expected distances).