Hi Jordan:
I really wish I could help more, especially considering that soft skins are my "thing" and especially US make vehicles used by other countries.
I can verify that Ford, Chevrolet and Buick sedans were "catalouged" official vehicles used as staff cars by Canadian Forces but you probably already have that info.
The problem that you run into with these civvie vehicles is that many of them, particularly if we are dealing with "Flag Rank" officers, were very much an ad hoc affair.
As you no doubt are aware, the Germans and the Japanese confiscated virtually the entire civilian vehicle park in the countries they conquered and occupied. This included, amongst other vehicles, thousands of US make vehicles that were either used by the troops or in the case of luxury vehicles by the Brass.
As the Allies took back these countries, the luxury vehicles that were left and running were in turn confiscated by high ranking Allied officers and put into their service.
The picture is further complicated by what Bruce describes. the outright purchase or requisition of large numbers of dealer's stocks, not only in the UK but also in Canada and the US and I suspect many of the Empire countries.
From a research point of view, these vehicles are a real headache as they for the most part are not carried "On the books" but are just folded into the mix.
Even if they are given official registration plates or military ID numbers on the vehicle, I doubt if we could ever completely track down these ad hoc sort of situations.
As a New Years Eve present, I can say that I would bet that the Canadian Forces had at least a few Packards on charge at some point in time during the 1939-1945 period. Just can't prove it tonight.
Happy New Year.
Bill