RE: EXI-450 discussions/mods/scale projects
You can mount the gyro either on the top or the bottom of the boom block. Either works fine but the top is less safe for the gyro in a crash. The bottom mount is less conveineint as you can't see the light on the gyro.
The bag as a spacrer should be fine. You just need some clearence for the gear as they are never perfectly round. Also I use the endpoints to adjust the gyro. They make more sense to me than the other way. Set it at 50% in both HH and normal and then adjust from there.