On my 76 525 the drivers and passengers side have different fixing positions and arrangement
For the drivers side, the seat has a vertical height adjustment, and there are 4 bolts that bolt the seat adjusting frame to the chassi.
To get to these bolts you probably have to raise the seat so you can get a socket in there . You do this by grabbing the lever near the centre tunnel and push the leaver forward, then with you body weight lean backwards, and the front of the seat ought to raise.
Then push the lever backwards and the back of the seat ought to raise. or vice versa. - mine is rusted stuck so I couldn't test.
Then simply unbolt the 4 bolts - M13 sockets. The seats are pretty heavy and take a bit to move.
On the passagers side, the sliding seat rails simply bolt to a raised part of the chassi. Slide the seat forward to expose the rear bolts in the channel guide, undo,
then slide the seat rearward to expose the front bolts undo those as well and the seat ought to be loose, 10mm socket for those