As for the 4D56 being too small... well, if you want a fast vehicle, then you'll be dissapointed. But don't think the 4M40 will make a huge difference either.

As for torque, the 4D56 TDI has about the same torque as the 6G72, but at 2000rpm. The 4M40 TDI generates about the same torque as the 6G74, also at around 2000rpm.

Diesel engines are not meant for speed (unless you have a newer CRD diesel)

'98 Galloper II Exceed 2.5TDI LWB 5sp Manual 5.29 Diffs