If you're talking about a bottom-mount, fixed bucket seat, like the Sparco Sprint, you're in for a very difficult job that I would not undertake.

Unless you're building a show car -- and you never intend to actually use the seats -- I wouldn't bother. I have a hard time believing any such solution will be safe enough to carry a passenger.

You'll have to fabricate structural bracing to attach the seat bases to the frame of the car, while figuring out how to maintain access to the fuel pump (remember, its under the OEM seat cushion). Furthermore, you'll lose the ability to fold either seatback down and you'll probably have to re-engineer the rear decklid that covers the rear hatch area.

Lastly, the angle of the seat back is nearly 90 degrees. So with the seats in an ergonomically safe orientation, a passenger will be sitting upright with his torso and head about a foot further forward than "normal". That means you or your front-seat passenger have to move your seat forward. Also, in a broad-side collision, I can see the rear seat passengers cracking their heads open against the B-pillar as their heads whip sideways.

Did I mention you'll have to incorporate a set of four-point harnesses as well?

In short, it's a bad idea all the way around and it will cost you a lot of money, time and effort to implement.