swop rear axle

Hello, I own a 1980 2.3 L petrol Bedford CF 280 ambulance converted to a camper-van that now has a 2.8L diesel engine from a Diahatsu 4 trak in it. Since the engine speed of the diesel engine is so much slower I need an axle ratio on the back axle to be much less than it is now ( 5.22 to 1).. I...