I just mated a 300DT with the 240 manual tranny. But I did it with both of them out of the car. I would bet that you'll have some struggles getting everything lined up underneath.

My car runs out fine but I need to update the differential in order to get more top end and longer acceleration between gears.

Don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed