The designers and engineers had access to the Camaro spoiler during the design process. If they had thought it looked right they certainly would have used it. Not to say that the designers always get it right (Pontiac Aztec) but they had a visual plan for the car and decided the spoiler wasn't in it. They made that pretty clear by putting the Pontiac letters and the 400 emblem right where the spoiler would sit.
I have bought and sold a bunch of Firebirds over the years and it never fails, someone has to do something to the car that is Camaro looking.
I think the worst thing is talking to a seller who has a fully restored 400 car and showing up and seeing the Camaro spoiler on the back. The first thing I ask is if they are willing to give credit for removing the spoiler and replacing the decklid. They don't want to talk to you much after that.
Having said that, I certainly believe it is anyones right to do what they will with their car. Just please don't call it restored if it has a Camaro spoiler, restified would be a better word.
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1969 Trans Am 4 speed 1 owner barn find-SOLD
http://www.carbuffs.com