As far as the backspacing for the wheels is concerned, I can't help you but, the Pontiac Parisienne and Chevrolet Impala are the same car under the sheet metal.
The Aussie delivered Parisiennes and Impalas are both sourced from Canada. The situation in Canada back in the '60's was different than that in the USA.
In the US, the full size Pontiac was the Catalina. There was no Parisienne model in the US until the late '70's. It was an entirely different car to the Chev Impala - even the bolt pattern of the wheels was different.
In Canada, whilst the sheet metal was different between the Chev and the Pontiac, all the mechanical, chassis etc were the same.
HOWEVER (whilst I'm pretty sure this doesn't apply in your case), the shape of the sheet metal on the rear mudguards can affect the size of wheel you can fit on Parisiennes and Impalas of the same year model. From memory, you can fit a bigger rear wheel on a '63 Parisienne/Laurentian that you can on an Impala/Bel Air of the same year.