You will get stuck having to smog the car--there are ways around that...mostly illegal, and I do not recommend any of them. That said, if you want to be legal in the smog arena, you'll have to swap in a '77 or later V8 into the 280Z. You can do dual cats if you want to or go with a single, which is easier to fab.
$600 is fair if the parts and procedure are top notch, period. Otherwise, it seems like too much money for a simple exhaust job.
As far as someone laughing at you abou the side pipes--there is serious consideration being given by several people, including Eric Neyerlin (sp?) about coming out with a kit. Side pipes are not my bag, but they don't look bad at all on a Z. You should talk to some street rod shops about them--they can do it right or they'll know about someone who can!
Davy