My '77 (which does have a pretty sweet aftermarket Alpine tape deck) only has speakers behind the seats on each side, you should see some holes in the panel. I think that was it for stock. I believe the 240Z only had one speaker!
Still no luck on making this kit for 280zs? I get voltage drops like crazy with mine, with the headlights and brake lights on (meaning foot on the brake) I'm reading about 13.8 at idle
If you do find a good shop, I'd be curious to hear about it. I live in Portland as well, and recently acquired a 77 280Z. Don't need to worry too much about carb tuning since its EFI, but it'd be nice to have someone to go to if I get in over my head ...
Sorry, that wasn't super helpful