"Fred, that sounds like a trunk lid from AMT Speed and Custom Division. They built complete cars and also sold individual pieces. The trunk lid replaced the stock steel outer skin from the factory decklid and bonded a fiberglass skin over the stock steel under-structure. With age, the fiberglass would delaminate from the steel due to rust coming from inside. This is what I heard anyway. There was one of these cars a couple of blocks down from my high school in the mid-70s".
Jon, That is exactly what it looked like. I don't remember the understructure being metal, but I'm sure it must have been. Interestingly, it also had two snorkels on the hood like those pictured, but I remember them being smaller. One was forward of the other, like the silver car in the center of that photo. No flares or side scoops though.