68 Barracuda Seat Issue

2 factors, the seat frames and the floor pan. Frames that are stressed from age /wear will lean. Cracks in frames also causes leaning. As for the floor pan, reinforcement plates at inboard mounting points should be present if this car had buckets from the factory. Where buckets replaced the OEM bench and those plates weren't added, leaning can result.
The most common cause is in the bucket seat frames. Pivot arm on back portion comes to rest on a stop post in the base. Look for issue there first.