Just don't forget to take them off before you leave. I used to put 1967 South Dakota plates over my regular plates on my 67 Dart at car shows. One time I drove about 30 miles after leaving a car show, stopped at a restaurant to have an early supper, got wondering so walked over to the window to look at my car and sure enough I didn't remove the 67 plates.
I now have it registered under the 1967 plates, supposed to be to and from car shows and parades etc., but have seen people license an older car this way and daily drive it.
Have a 1954 unissued South Dakota trailer plate that I replace the teardrop plate with at car shows. Ebay is a great source for finding plates, and the prices don't seem too far out of line. Either that or nobody wants South Dakota plates.
Aaron
If I could shut my brain off, I could save myself alot of time, money and effort.