Hi,
On a classic looking car you shouldn't have too much trouble from the law, but may you fail at MOT time if the examiner goes by the book!....you can always keep some reflective ones to use at that time.
My old 300ZX had small square number plates with undersize lettering and I never had any problems from the Police or mot station.They are clamping down on illegal no. plates at the moment though!...So you may have to have them made as "show plates" from one of the many companies on the web.
Cheerz,
JEZ