'68 Valiant Rear Seat Removal

The bottom of early seat backs had only couple of metal tangs poked through slot of the frame and bent. Gravity, etc.., would cause the back to drop down some, then its in the way of moving the seat bottom rearward. Try to raise the back a little, then push the bottom rearward.
They did change to bolts at the bottom loops of back frame at some year model. You can see where the back is fastened in corners here. Socket and extension will loosen the bolts.
1967 1968 1969 Dodge Dart GT Swinger Plymouth A-Body rear seat 2 door hardtop | eBay