Why cut the tenon long first? If you are real lucky, the barrel will index the way you want it, and you're off to finish the rest. In the world of the rest of us, you will have to start cutting the shoulder to get it to index, which you will then have to shorten the tenon, and finish. It makes things a lot easier if the action face has been trued, and the recoil lug surface ground. There will be take-up when tightening, so don't fit it dead nuts in the lathe.
