That 4-door conversion looks really interesting, probably a little more than I'd want to tackle, though. Thanks for posting that MPC 1949 Mercury police car, Jon. Does it look a little "hot-roddie" to you? Does to me. Lee, I'll take a look for that build-up, too.
Let's try to stay in topic, guys. Now, since I am planning on building the 1949 Mercury as a police car, it sounds like the AMT kit is the way to go. It might not be perfect, but I figure with the light & sirine from the 1957 Ford Custom it'll be passable. I don't know how popular these cars were with departments, but it seems logical. The only department that I know of that used them is Radiator Springs!
Weather it was or wasn't is irrelevant at this point. The original question has been answered; and I learned a few things along the way about 1949 Mercury cars, and their availability as models. Now it's time to move onto more important things: like does me LHS carry them? I did find the AMT & Revell/Monogram kits listed at Hobby Link. It's easier to search for something of you know what to look for.