The common shaft would have flats milled on the face to align all the butterfly valves. As long as you accurately do this, the angles of the valves should be extremely close to eachother.
Good point 1 Fast Z. It's almost a project that could be thrown together over one weekend, except for the gun drilling which I do not have access to.
Your project is obviously for race, where as mine is a daily driver. How well ITB would work for that, I have no idea. I suppose if it doesn't run too well for my purposes, it could be sold to a member here.
All the fun is in building these things anyway!