Some G81-3000 with doubleshots do have space bars with Cherry G81-specific stabilisers that are incompatible with Cherry MX.
The G81-7000's space bar is 6u (not 6.25u) and the switch's stem is not centred, but it should fit other keyboards in the G80-series with 6u space bar. I've got one on my Phantom, but it is customized.