When General MacArthur arrives in Japan, he is seen riding to his headquarters in a Chrysler New Yorker automobile of either a 1946-1947 or 1948 model. They all sported the same body style in those years. So he wouldn't have been riding in one of these Chryslers.
As I understand it, Chrysler introduced its post-war models in late 1945. Just speculating here, but is it possible that Chrysler gave General MacArthur an advance copy of their upcoming 1946 model of the 1946 New Yorker? I know that during WWII Chrysler did a lot of stuff for the Army - essentially all of Chrysler's production was devoted to producing tanks and other military vehicles as well as radar and the like. Maybe since they were so thick with the Army that they gave MacArthur a special advance edition of their upcoming 1946 Chrysler New Yorker.
Since Chrysler would not have known the war was going to end in August 1945 I doubt if it would have tooled up for civilian cars in time for MacArthur's entrance into Japan in September 1945.