Is the chassis interchangeable on a 1978 4-door caprice and 1977 impala 2 door?

No, the chassis are not interchangeable between a 1978 Chevrolet Caprice 4-door and a 1977 Chevrolet Impala 2-door. While they share some similarities as GM B-body cars, the wheelbases are different (Caprices were generally longer), and the chassis structures, particularly the floorpans and unibody reinforcements, are designed specifically for their respective body styles and door configurations. Trying to swap them would require extensive, impractical modification.