I found the thread by Fikshun where he put Celica seats in his FC but all the links are dead and there aren't even any pics of his brackets. Does anyone still have these saved or has anyone done this since?

I really need a lower seat since my head hits the ceiling...my seats are kinda raggedy too.

 

Putting any seats in the car- save the stockers- is going to be an exercise in fabrication, unless you go aftermarket (not OEM like the Celica) and they provide custom brackets.

The driver's side is relatively simple because the mounting points on the chassis are symmetrical (front to back and side to side).
You'll gain the most headroom by eliminating the sliders and just using flat bar stock but this can present problems in two areas.
First will be installing the rear mounting bolts...depending on where you locate the seat, you may not be able to access them. You can get around this by putting studs in the rear mount location and just use nuts...much easier to fish them on blind as opposed to trying to locate and start bolts.
The second problem will be adjustability...there ain't gonna be any.
The seat may fit you but others may have problems.
This may or may not be an issue for you.

The passenger side is a whole 'nother story.
The seat base on the stocker is quite elaborately malformed to accommodate the odd mounting points in the chassis (remove the stocker and you'll see this immediately).
Getting any other OEM seat to fit here will just be an exercise in ingenuity.
Fortunately, headroom is not such an issue on the passenger side, so it can be done, albeit not easily.

Sorry this does not specifically address your question about the Celica seats but it's generally pertinent.