The problem with most of the "frames" that are available for the Supra is that the seat sits on top of the runners as previously mentioned. In addition, a lot of the designs have bars joining the left and right rails of the frame at the runner height which prevent the seat being lowered anymore, as the base would foul these bars.
I ended up making my own custom rails as the Cobra ones were absolutely atrocious, even after they'd sent me 3 different sets (each time the design was different but on no occasion did all the holes line up and on one set the welding was quite frankly shocking, even if it had of fit in the car I'd have refused to use them).
As it happens that did me a favour as I could replicate the design of the original seat rails where the base of the seat can "drop" between the runners, though this would not be acceptable for all seat designs as the seats themselves then need enough lateral strength in the even of an impact.
Wez: That Nagisa Auto advert must be using reference images as those rails can't fit a Supra, I'd ask for some more photos before purchasing, the inside rear pickup point is wrong.