340 xhead spring shimming, height and pocket question(s)

This is just my opinion. If both surfaces are final machined and you keep track and replace the gaskets in the same orientation as when mocked up then yes. You can torque them short of your final torque spec in mockup. I save old gaskets ( in good condition) for mocking things up.
If you end up with plenty of coil bind clearance with the springs you end up using you can skip it. Unless you want to do it for the experience which I completely understand.