Q:

How to install an electric motor-driven radiator cooling fan instead of the belt-driven one? 

I want to know about how to install an electric motor-driven radiator cooling fan instead of the belt-driven one. 

Vehicle: 1998 Mitsubishi-fuso  fm617
Engine: Mitsubishi 6d16 diesel engine (radiator fan driven by belt in original)
Type: truck

A:

Not to be crass here, but you should just find one on the internet (Flex-a-lite makes a bunch of different models) which fits and follow their directions.  I put one of their Monster Fan kits on my '04 Suburban.  It worked very well.  
Measure the size of your radiator and go from there, as I doubt they make one specific for your application.  The ones which have their own temperature probe and controller will be your best bet.  There would be no way to tell you how to install it over the internet ... not any better than the instructions, anyway.