Rails and members remain.
The idea is to drop the mounting plane for the seats. Not necessary, or at all advised, to alter chassis reinforcement structure.
Prior to undertaking this modification, you can lower seats some by removing some spacers that were OEM, and some owners pick one seat position, and then remove seat slider rails and anchor seat in desired spot.
Most of these cars are one driver cars so seat adjustment fore and aft is not a needed option.
Larry
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