I've followed along the same lines as Q, but went with a 2.5" exhaust into Flowmasters, and they dump before the rear end.
Then, on top of that, I put an Alpine head unit in the glove box, a Kenwood 280W 2/3/4 chnl amp and an Alpine 180W 2chnl amp in the trunk along the support brace (convertible) and an 8" Free air Kicker sub mounted there too partially using the area behind the seat where the Top Motor goes (filled with Polly Fiber fill) as a box. Two Infinity 6x9's (re-making the custom enclosures) firing forward into the area where the top folds (only open area into the cabin) Then I hav two Aura Bass Shakers under the front seats (aura.com) 25w per. And a high powered 10x4" in the stock dash speaker location, and soon to install the two 4x6" Infinity speakers in the kick panels in the stock speaker location with the stock speaker grills.
All in all.... theres a whole lotta shak'n going on! What you don't hear over the 455/hooker/Flowmaster combo, you feel in the bass shakers... so you at least know the tunes are rumbling while your cruising at 80... er.. ahh.. I mean 60... yeah... 60...
I have to update the pix on my web page... but you can get the idea of the start of the install. brettliukkonen.com and go to the firebird page.
Or maybe Bjorn will post the link to Chris Reynolds stereo install in his 69. Now that is nice!
It's all good... and loud! - Brett - 67 Drop Top 455!