The main thing to realize is that the LED 1157 replacements will give the tail light a "dot" of concentrated light versus the "filling" effect of an incandescent. The LEDs tend to concentrate light at a much narrower point. When you have a 66 - 67 GTO with wide tail lights, the LED bulb does not "fill" the area well.
Wouldn't this qualify for "filling effect"? 39 LED's.