Well, maybe the floorpans and inner rockers on my very early car are a little different than the later cars, but I had no problems installing my new Monte Carlo's (they are called NurnbergR from BFtorino in europe)in my pushbutton.
I used the original sliders. I order to line up the threads on the underside of the seats I had to drill new holes in one slider and fab and weld a bracked to the side of the other - that moved them outward by about an inch. No clearence problems with the handbrake box and they move freely all the way back - and forth.
I'm 188 cm (6'15")and still have about 2" clearence to the roof.
Not too good a picture of the pass side seat installed.