Like @DLane said, you can certainly extend the length of the current assembly to get the needed length.
As stated, you always want a PVC to metal connection to have the PVC end be a male connection, so I’d recommend adding a coupling on the right side, or possibly another union to make the whole system removable if needed…
Then I would grab 4 of these push-on x male adapters, 1 for the 90 on the left, 1 for the union on the left, 1 for the bottom of the backflow, and one for the newly installed coupling or union on the right. Then a couple sections of pipe to mount the flow meter with a couple push on couplings and you’re set. Won’t be the prettiest thing, but it will work just fine…