X2 on panZZers comments.
I would put a 5 cylinder in before I turbo the 240.
Depending on the year of your 240 it has 65 or 72 HP.
If it is a 65 HP, You can pep it up a little with the cam from a later 616.
If you using a 617 get one that is 1980 or later.

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76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.