The original metal taillight panel on my gt350 was cut out to accomodate the cougar tailights. The fiber glass Shelby taillight panel is screwed on to the the areas above and below the original concave metal taillight panel. The fiberglass panel fits nicely in the space bordered by the 1/4 panel extensions, valence, and deck lid.
I'm not sure if I've answered you questions. Let me know if you need more info.
wjj