If you are planning a rifle build, and using a button rifled barrel, I`d go an 8" twist.
If you use a cut rifle, then a 9 might work fine, but a 8.5" would be my first pick.
I shoot a 6.5/284 with 8.5" twist and the 142 Sierras are right at home.
It is cut rifled.
The button rifled barrels cannot hold twist as accuratly as cut barrels and run up to a 1/2" slower twist than ordered.