I've seen some pretty bad 55 Chevy’s for sale as parts cars, try a search on goggle. You might be able to buy a rusted fender that you could cut the section out for the tail light. Or you could set the 55 Chevy tail light assembles on with duck tape use some 1/4" rod blend it to your fender with metal. I have been thinking of 39 Ford tail lights in my 51 Plymouth.

Richard