When Willy is changing license plates on the pickup it is a Chevrolet. As he drives off from the gas station it is a Studebaker with no plates. Then later the Studebaker has the foreign plates Willy had installed.
Private jets such as the one used here typically cruise at altitudes of 30,000' and higher. The temperature at that altitude would be at least -30 degrees and the atmosphere would be far too thin to support life. Finch stowed away in an unheated and depressurized access area. He would have been dead long before reaching their destination.
As part of the prep for making Dolan believe he is on a foreign island, Willy is seen changing the front license plate on a pickup truck from a Georgia plate to that of the island. The state of Georgia doesn't use front license plates - only rear plates.
At about ten minutes in, Barney and Phelps put on masks and knock out their passengers with gas. You can clearly see holes in their masks.
At 30 minutes 40 seconds, the face of Peter Graves' stunt double is seen clearly.
Before Barney fills the jet with knock out gas, he and Jim are seen putting on masks, these masks appear to be oxygen masks, there are holes in the masks that they are using. They would not prevent both Jim & Barney succumbing to the effects of the gas themselves.
At several points during this episode mountains are glimpsed in the background. Southern Georgia has no mountains and certainly not on its islands.