That's real bad man.
Is it fixable, yes. Will it be pretty expensive, yes.
i told you about this before when we did the tires on your se's that one time.
thisTake seat off. Mount rails to a 1/4" steel plate.
Put plate and rails over hole in floor.
Put seat back in.
$15 steel plate 2'x2'
thats the cheap way till you find.someone to weld you.a new floor.
and this tooYes it cheap if your step mother does it, but it is better to have a professional does and thats why it cost more cause he knows what he is doing.