71 Demon seat slides back

Wd might help if that is the problem.
To remove the seats they have studs through the floor. Four 1/2” (or 9/16” I forget) nuts you will have to remove from under the car.
Always put a stand under the front of your car when crawling under there.
Torsion bar is the only thing holding up the car.
Not likely a torsion bar will break when you are under there but it is possible just not probable.