Alfred, if you check the Berger site, you'll see the recomended twists of "their" bullets which are a longer VLD requiring a tighter twist than most conventional bullets. The 168 VLD requires a 10 twist or better and the 180 VLD requires a 9 twist or better. The Berger recommended twist is *probably* conservative. A 180 VLD would *probably* work in a 9 1/4 twist depending on velocity, barrel length, and air density. A 160 gr NAB would work in a 10 twist.
If I was going to do a custom build, I would probably get an 8 twist or tighter for some of the longer high BC wildcat or custom bullets... especially the monometals.
Barrel length will probably boil down to about 25-30 fps per inch.