RE: Black Magic v2 builder's thread! *Updated*
Gene, absolutely locate the mount first. If you build it straight and take your time, everything will line up perfectly.
OS vs Webra: No question in my mind. OS 140rx all the way.
A Budd mount is fine, but you'll need to use the nose ring former. When you do this, make sure you reinforce this former with strips of glass or carbon on both sides.
-Mike