You've probably yanked the 4runner seats out by now and seen that they're held on by 4 bolts to the frame.
I'd think that, assuming the celica seats are not power, I'd see about adapting the 4runner slide rails to the celica seats.
Maybe you're really lucky and the celica's brackets (the 4 point anchor) are in roughly the same pattern as the 4runner's.
I love my seats, but the lumbar is in exactly the wrong place for me... too high up. But then again I'm 6'4" and most of that is leg. God I wish the wheel telescoped.