Homer the balance C.G. does not change moving the fire wall back. Best way I found to move a fire wall is redraw where it needs to be on the plans. Mount or standoffs need to be on the engine, then set it on the plan so the back of prop is 1/16" ahead of the cowling or the place its shown on the plans. Square the engine up as close as possible and mark the back of the mount on the plans, that will be the face of the new location of the fire wall.

Leroy