I will say one certain difference between a 66 and a 65 qt is the taillight opening D shape. The 65 being larger. Bumper inset area something to dble check too.
I don't think the shape is different. The taillights should interchange.(between the 65 and 66) The quarters are actually 2 piece. The outer skin and the inner trunk rail should be seamed under the chrome that runs from the back glass to the taillights. If you notice the 64's with the small round taillights have the same outer shape but a wider deck lid and a narrower sized inner quarter.