I sent ya an email explaining, but since there's so much interest I'll post it here, as well.
I've seen it done 2 ways.....the way I did it and the way others have done it. They are essentially the same, but here goes....
I'm not familiar with the 3rd gen seats, I'm using 4th gen seats, but I've heard that 4th gen seats are a direct bolt in for 3rd gens.
I got a set of power leather buckets out of a 97 Z-28....seat brackets would have same mount holes as a 4th gen Firebird or TA. The front holes on the 4th gen seat brackets line up perfectly to the front floor pan holes in first gen cars. The trick is the rear and it's essentially just modifying the "feet" on the 4th gen brackets.
I wanted to maintain the 6 way power adjustment so I kept everything from the 4th gen seats. By doing so I also gained about 4 inches of leg room from the first gen seats.
Basically, I cut the feet off the rear seat bracket legs. I made bases out of 1/8th inch x 1 inch flat bar by cutting 2.5 inch lengths. I then drilled a hole in the center of one end and loosely bolted it to the floor. I then positioned the seats in place and bolted down the fronts using the original holes and lined up the rear feet under the legs of the seat brackets and tack welded them in place. I then unbolted the seats and finished welding the new "feet" to the legs of the seat brackets.
The passenger seat does require a 1 inch spacer to lift the front of the seat so it sits level. I used a 12 inch length of 1 x 2 square tube as a spacer, drilled 1/4 inch holes and used longer bolts to bolt the front of the seat down.
I also ran a power wire to the driver seat using a switched source under the dash.
The rear seat requires a little more work. You have to make a bracket to bolt the seat back to, then make another bracket to facilitate the upper latch mechanism to if you want the seat functional. The seat bottoms fit between the trans tunnel and door panels, but have to be trimmed on the bottom to fit over the torque boxes made into the floor of a first gen. To do this you simply cut the metal support rod about 2 inches above the lower corner then cut it about 4 inches forward of the corner on the bottom and remove. Use a straight piece of metal rod to replace the piece you cut out. Basically the outer bottom corner of the seat bottom will now be straight "/" instead of "L" shaped. You then have to make a bracket to botl the front of the seat bottom to. I used GMs design....saw how it was installed in the 4th gen and recreated the same type bracket for the front edge of the seat to mount to. It's held in place with 1 bolt through a single metal loop on the bottom of the seat.
Clear as mud?
Hope that helps! The front seats are a BREEZE the rear is a little more work, but still not hard!
