Classic Sedans, Climate Control

Me again. I asked a question about my 1978 300SD with a bad engine. I can get a really great deal on a low mileage 300TD engine and would love the increased power. My dad thinks we can just swap them out as long as they are the same size. Is there a size difference? It is hard to tell by looking and is really hard to measure.
Or should I just keep looking for a nonturbo engine?
(Don"t know how long the turbo engine will be available so please tell me something)

My 86 560sel sputters as i first step on the gas. At about 1500 rpm the engine picks up and runs like the beast its supposed to be. When i come to a complete stop and the car is still in gear the engine misses constantly. If i just put it in neutral it revs a little and runs smooth again. I just bought this car for next to nothing and put heat in the gas tank, along with a good amount of the recommended "premium" fuel, and it seems to have helped a little bit so far.