are the heads different? if they are (i.e. 2l has better flow) ud be better off swapping the whole engine to get decent gains. in theory, swapping the head alone would be better than swapping bottom end (providing the new 1 flows better) than the swapping bottom end, since power/torque is proportional to mass airflow through the head. but doing both (i.e. swapping whole lump) is better still