Not all will fit. A lot of 4x6s have a tweeter that sticks up too high and requires you to grind away a little on the backside of the stock grills. In firebirds and 91-2 camaros, you may also run into fitment problems with speakers that have large magnets. Pre 91 camaros have a lot more room. In pre 91 camaros you can even fit 4x6" plate speakers with no modifications, although in firebirds there will be some fairly major modification necessary to get them to fit the driver's side.
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The IROC Homepage
<A HREF="http://www.rit.edu/~jli4307/camaro" TARGET=_blank>
View the restoration of an 85 IROC</A>
"I didn't know a bored out Ford could go so slow" -Shenandoah