If the rear seat fits and can be installed securely, I can't see why it wouldn't be legal in any state of the country.
As to whether it fits easily, that's difficult to answer. There are various things to consider.
1. The seat will be designed to fit between two fixed side panels, not two opening doors. That means the corners of the cushion (the lower part) won't be rounded off to assist rear seat access.
2. The rear parcel shelf may be higher in the Monaro, which means the top of the seat will stick up above the sedan rear shelf.
3. The design of the rear bulkhead may be different. Did the Monaro have the rear port-hole - I can't recall. The six bolts and two hooks that locate the rear upright in the sedan may be different.
4. The sedan upright was a three piece design, which made it easier to install. Is the Monaro's similar or is it one piece? ( I suspect it is in sections).
You would need to take some careful measurements before taking this one on. Perhaps compare the rear seat from a sedan with the Monaro's by taking it out of your car and putting them side by side to compare.
