Convince me to put a TKX in my Dart
This is an NOS Petty trans cross member in blue that goes to the torsion bar support
To the bottom right of the red box is a NASCAR torsion bar adjuster screw.
The motor plate is for a small block obviously. It may be the same one in that Nascar Ontario Speedway 1977 poster that is of an old Petty Charger used for Winston Cup as a promo car. Pictures show it was just slapped together, not drivable. Detail from poster in that second picture