which deisel to fit?

[uley 319]

Hallo, sorry to write this in English, but my German is not good. Can anyone help with advice on which Mercedes deisel engine to fit to my L319 fire engine? I bought my 319 about two years ago and have converted it to a basic camper keeping the fire engine bench seats, pegboard lining etc to retain its character. We have just come back from from France and want to change the engine for a deisel for reliability, speed up hill!, and economy. I have heard that the engine and gearbox from a 3/207d works well in the 319 and is not very difficult to do.
Does anyone have experience of this, and what do you need to change to make the new engine and gearbox fit, clutch pedal, gear stick etc? What sort of economy and speed can you get with a deisel engine/gearbox from a 307d? Should I buy a whole van or would just the engine and gearbox do? Any advice and help would be very much appreciated. Best wishes to all, Jamie.