Torquemonster is right, the 4d56 is a direct bolt on and the head has a better design. Be careful not to get a head from china, or any 4d55/4d56 parts for that matter, I have only heard horror stories. Also make sure you get the age apropriate head for the vehicle. For example you don't want the head for the common rail 4d56.

Spence
1985 Mitsubishi PU 2.3 Turbo Diesel, 13" lift, Watercooled turbo, 4D56 Head, Atlas II,Fox Coilovers, Elockers,

1983 Mitsubishi PU 2.5 Turbo Diesel 4x4, fully restored

2003 G500, sliders, beadlocks, roofrack, ladder etc.