Both are eligable for collector plates. The '47 is a direct deal. You go in with the title and they will either give (sell) you regular plates or you can go with collector plates or go on Ebay or here:
Old License Plate (I've gotten 2 plates here) and get actual year of issue plates. On your Cobra you will have to get it state inspected which means they look at it and agree that it looks like the year you want to register it as. Mine was originally a Shell Valley ASV 1998 with a Kansas title. I titled it as a 1966 Shelby Cobra and run 1966 plate (if you title it as a collector you do not need a front plate, if not, you do). After the first registration in WA, which you will pay for, you do not pay again for annual registration if you run vehicle as a collector vehicle. The law says collector vehicles can only be driven to shows and gatherings, not as a regularly driven vehicle. I have never heard of anyone getting stopped and asked about this. There is a vehicle registration office (they have contract offices in WA) in Gig Harbor, about 15 minutes from Post. The girls there have done a bunch of these and are the easiest ones around to work with. The registration office will give you a form and sign it and you take it out to the inspection office, about 10 minutes from Post where they look at your car for 2 minutes, BS with you about Cobras for 15 minutes and send you on your way. They do not look at anthing on the car, safety wise.