First you gotta find a 603 engine in good shape. This will be more difficult than you would believe, my favorite MB yard didn't have any at all last time I asked, just "cores" with something serious wrong with them (typically bad heads or bent rods).
It can be done, but it ain't gonna be cheap!
You will need engine, transmission, rear end, probably driveshaft, new fuel lines since the diesel ones are different, different tranny control system, and a different computer (yes, the 601/602/603 turbo has a computer...).
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!