While you have it jigged up and aligned you could also drill and tap the 8 holes used to bolt the cam box to the breech casing. Once the cam halves are split it is abit more difficult to ensure these holes are in the correct location. Just and idea.
Outstanding idea, beats the way I was going to approach it.
If I get my box cam done before you guys are ready to do yours you are welcome to use the fixture, as long as you don't mind paying shipping. That goes for anyone else that may need it.