Boatdoc: Your 73 shuld have the long rectangular tail lights, if so then take a look at Champion Trailer, they have some sealed LED Tail Lights that might work. I also saw some LED boat lights at Harbor Freight the other day that might work.
I had to remove the plastic housing on my 73 Argosy, as it was startig to break up. I installed Grote Versalight Double Pod Surface Mount Units. Grote Industries: RV, Marine, and Utility Trailer Lighting
I posted a picture somewhere on this site. I had a problem with the large rectangular cutouts that the factory made in the skin. I fabricated cover plates, and rivited them on and then installed the light assemblies.
I plan to convert the Grote lights to LEDs in the future. There are posts on this site of how this has been done.
Just my thoughts, Sam Halderman